UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             Washington, D. C. 20549

                                  FORM 10-KSB/A

                                (Amendment No. 1)

[X]  ANNUAL REPORT UNDER SECTION 13 OR 15(d) OF THE  SECURITIES  EXCHANGE ACT OF
     1934

                   For the fiscal year ended December 31, 2004

[_]  TRANSITION REPORT UNDER SECTION 13 OR 15(d) OF THE SECURITIES  EXCHANGE ACT
     OF 1934

          For the transition period from _____________ to _____________

                         Commission file number 1-31398

                        NATURAL GAS SERVICES GROUP, INC.
                 (Name of small business issuer in its charter)

             Colorado                                            75-2811855
(State or other jurisdiction of                               (I.R.S. Employer
 incorporation or organization)                              Identification No.)

                           2911 South County Road 1260
                              Midland, Texas 79706
               (Address of principal executive offices) (Zip Code)

                                 (432) 563-3974
                (Issuer's telephone number, including area code)

         Securities registered under Section 12(b) of the Exchange Act:

                           Common Stock $.01 Par Value
                                (Title of Class)


                        Warrants to Purchase Common Stock
                                (Title of Class)

       Securities registered under Section 12(g) of the Exchange Act: None


         Check whether the issuer (1) filed all reports  required to be filed by
Section 13 or 15(d) of the  Exchange  Act during the past 12 months (or for such
shorter period that the  registrant was required to file such reports),  and (2)
has been subject to such filing requirements for the past 90 days. Yes X  No
                                                                      ---   ---

         Check if there is no  disclosure  of  delinquent  filers in response to
Item 405 of  Regulation  S-B contained in this form,  and no disclosure  will be
contained,  to the best of the  registrant's  knowledge,  in definitive proxy or
information statements incorporated by reference in Part III of this Form 10-KSB
or any amendment to this Form 10-KSB. X
                                     ---

         State issuer's revenue for its most recent fiscal year: $15,958,199

         The aggregate  market value of the voting and non-voting  common equity
held by non-affiliates  at March 23, 2005,  computed by reference to the closing
price of $10.76 per share on the American Stock Exchange, was $47,676,914.

         The number of shares  outstanding  of each of the  issuer's  classes of
common equity on March 23, 2005, was 6,765,764.

                       Documents Incorporated by Reference
                                      None

         Transitional Small Business Disclosure Format Yes    No X
                                                          ---   ---


EXPLANATORY NOTE Natural Gas Services Group, Inc. is filing this Amendment No. 1 on Form 10-KSB/A to its Annual Report on Form 10-KSB for the fiscal year ended December 31, 2004, as filed with the SEC on March 30, 2005, for the purpose of amending and restating in its entirety Item 11 and Item 13 of such Annual Report on Form 10-KSB, as set forth below. As a result of this amendment, Natural Gas Services Group, Inc. is filing as exhibits to this Amendment No. 1 on Form 10-KSB the certifications pursuant to Section 302 and 906 of the Sarbanes-Oxley Act of 2002. This Amendment No. 1 on Form 10-KSB/A does not change our previously reported financial statements and other financial disclosures. Items included in the original Form 10-KSB that are not included herein are not amended and remain in effect as of the date of the original filing. Additionally, this Form 10-KSB/A does not purport to provide an update or a discussion of any other developments subsequent to the original filing. 2

Item 11. SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT AND RELATED STOCKHOLDER MATTERS The following table sets forth, as of March 18, 2005, the beneficial ownership of our common stock by (i) each of our directors; (ii) each of our executive officers; (iii) all of our executive officers and directors as a group; and (iv) each person known by us to beneficially own more than five percent of our common stock. Except as otherwise indicated below, each of the individuals named in the table has sole voting and investment power, or shares such powers with his spouse, with respect to the shares set forth opposite his name. Percent Name and Shares of Common Stock Beneficially Address Beneficially Owned Owned - ------------------------------------ ---------------------- ------------ Wallace O. Sellers 693,159(1) 10.2% P.O. Box 106 6539 Upper York Road Solebury, Pennsylvania 18963-0106 Charles G. Curtis 80,500(2) 1.2% 1 Penrose Lane Colorado Springs, Colorado 80906 Paul D. Hensley 426,829 6.3% 3005 N. 15th Street Broken Arrow, Oklahoma 74012 William F. Hughes 247,000(3) 3.6% 42921 Normandy Lane Lancaster, California 93536 Wallace C. Sparkman 167,691(4) 2.5% 4906 Oakwood Court Midland, Texas 79707 Gene A Strasheim 8,500(5) * 165 Huntington Place Colorado Springs, Colorado 80906 Richard L. Yadon 299,183(6) 4.4% P.O. Box 8715 Midland, Texas 79708-8715 Ron L. Bingham 6,000(7) * P.O. Box 945 Lewiston, Michigan 49756 3

W. Randy Larkin 12,000(8) * 5609 Heartland Midland, Texas 79707 S. Craig Rogers 14,250(9) * 14732 Bluestem Ave Gardendale, Texas 79758 Earl R. Wait 75,520(10) 1.1% 5102 Teakwood Trace Midland, Texas 79707 Scott W. Sparkman 519,467(11) 7.7% 1604 Ventura Ave Midland, Texas 79705 Charles L. Barney 936,274(12) 13.4% 952 Echo Lane, Suite 364 Houston, Texas 77024 RWG Investments LLC 369,000 (13) 5.5% 5980 Wildwood Drive Rapid City, South Dakota 57902 Babson Capital Management LLC 651,700(14) 9.6% One Memorial Drive Cambridge, Massachusetts 02142-1300 All directors (and nominees) and executive officers as a group (12 persons) 2,550,099 (15) 36.5% - --------------------------------------- * Less than one percent. (1) Includes 196,091 shares of common stock owned by the Trust under Deed of Wallace O. Sellers, dated June 21, 1991, 196,091 shares of common stock owned by the Trust under Deed of Wallace O. Sellers, dated June 22, 1971; options to purchase 2,500 shares of common stock at $3.88 per share, options to purchase 2,500 shares of common stock at $5.55 per share, and options to purchase 2,500 shares of common stock at $9.34 per share held by Mr. Sellers; and 158,600 shares owned by Mr. Sellers' wife. The trustee of each trust is an unrelated third party. Mr. Sellers' wife is a contingent remainder beneficiary of one trust and a beneficiary during her lifetime of the other. (2) Includes options to purchase 2,500 shares of common stock at $3.88 per share, options to purchase 2,500 shares of common stock at $5.55 per share, options to purchase 2,500 shares of common stock at $9.34 per share, and warrants to purchase 40,000 shares of common stock at $3.25 per share. (3) Includes 180,500 shares of common stock and a warrant to purchase 60,000 shares of common stock at $3.25 per share owned by the William and Cheryl Hughes Family Trust, an option to purchase 2,500 share of common stock at $5.55, and an option to purchase 2,500 shares of common stock at $9.34. (4) Includes 105,691 shares owned by Diamente Investments, LLP, a Texas limited partnership of which Mr. Sparkman is a general and limited partner. (5) Includes options to purchase 2,500 shares of common stock at $5.55 per share and options to purchase 2,500 shares of common stock at $9.34 per share. 4

(6) Includes warrants to purchase 9,365 shares of common stock at $2.50 per share, warrants to purchase 5,318 shares of common stock at $3.25 per share, options to purchase 2,500 shares of common stock at $5.55 per share and options to purchase 2,500 shares of common stock at $9.34 per share. (7) Includes an option to purchase 6,000 shares of common stock at $5.58 per share. (8) Includes an option to purchase 12,000 shares of common stock at $7.50 per share. (9) Includes warrants to purchase 1,125 shares of common stock at $6.25 per share and an option to purchase 12,000 shares of common stock at $3.25 per share. (10) Includes an option to purchase 15,000 shares of common stock at $3.25 per share. (11) Includes an option to purchase 3,000 shares of common stock at $7.50 per share, and 475,000 shares of common stock and warrants to purchase 21,467 shares of common stock at $2.50 per share owned by Diamond S DGT, a trust of which Mr. Sparkman is a co-trustee and co-beneficiary with his sister. (12) Based on Amendment No. 1 to Schedule 13D filed with the SEC on March 22, 2005, Charles L. Barney, the sole indirect owner of CBarney Investments, Ltd. and Mark X Energy Company, reported beneficial ownership of 707,974 shares of common stock and warrants to purchase 228,300 shares of common stock. Mr. Barney reported shared voting and dispositive power with (i) CBarney Investments, Ltd. with respect to the 600,674 shares and warrants its owns and (ii) Mark X Energy Company with respect to the 335,600 shares and warrants it owns, due to his ownership control of those entities. (13) Includes a warrant to purchase 15,000 shares of common stock at $6.25 per share, 245,000 shares of common stock owned by RWG Investments LLC, and 82,000 shares of common stock owned by G Five Development LLC. RWG Investments LLC is a limited liability company owned solely by Roland W. Gentner. G Five Development LLC is a limited liability company, the beneficial owners of which are Roland W. Gentner, his spouse, and his three sons. (14) As reported in Schedule 13G filed with the SEC on February 8, 2005, Babson Capital Management LLC, in its capacity as investment adviser, may be deemed the beneficial owner of such shares which are owned by investment advisory client(s). Babson Capital reported sole voting power with respect to 641,400 shares; shared voting power with respect to 10,300 shares; and sole dispositive power with respect to 651,700 shares. (15) Includes options to purchase 78,000 shares of common stock and warrants to purchase 137,275 shares of common stock. 5

Item 13. EXHIBITS The following is a list of all exhibits filed as part of this Form 10-KSB/A: Exhibit No. Description - ----------- ----------- 2.1 Purchase and Sale Agreement by and between Hy-Bon Engineering Company, Inc. and NGE Leasing, Inc. (Incorporated by reference to Exhibit 2.1 of the Registrant's Current Report on Form 8-K dated February 28, 2003 and filed with the Securities and Exchange Commission on March 6, 2003) 3.1 Articles of Incorporation, as amended (Incorporated by reference to Exhibit 3.1 of the 10QSB filed and dated November 10, 2004) 3.2 Bylaws (Incorporated by reference to Exhibit 3.4 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.1 Form of warrant certificate (Incorporated by reference to Exhibit 4.1 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.2 Form of warrant agent agreement (Incorporated by reference to Exhibit 4.2 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.3 Form of lock-up agreement (Incorporated by reference to Exhibit 4.3 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.4 Form of representative's option for the purchase of common stock (Incorporated by reference to Exhibit 4.4 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.5 Form of representative's option for the purchase of warrants (Incorporated by reference to Exhibit 4.5 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.6 Stockholders Agreement, dated January 3, 2005 among Paul D. Hensley, Tony Vohjesus, Jim Hazlett and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 4.3 of the Registrant's From 8-K Report, dated January 3, 2005, as filed with the Securities and Exchange Commission on January 7, 2005) 6

Executive Compensation Plans and Arrangements (Exhibits 10.1, 10.24, 10.25 and 10.26) 10.1 1998 Stock Option Plan (Incorporated by reference to Exhibit 10.1 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.2 Asset Purchase Agreement, dated January 1, 2001, between the Registrant and Great Lakes Compression, Inc. (Incorporated by reference to Exhibit 10.2 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.3 Exhibits 3(c)(1), 3(c)(2), 3(c)(3), 3(c)(4), 13(d)(1), 13(d)(2) and 13(d)(3) to Asset Purchase Agreement, dated January 1, 2001, between the Registrant and Great Lakes Compression, Inc. (Incorporated by reference to Exhibit 10.14 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.4 Amendment to Guaranty Agreement between Natural Gas Services Group, Inc. and Dominion Michigan Production Services, Inc. (Incorporated by reference to Exhibit 10.3 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.5 Form of Series A 10% Subordinated Notes due December 31, 2006 (Incorporated by reference to Exhibit 10.8 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.6 Form of Five-Year Warrants to Purchase Common Stock (Incorporated by reference to Exhibit 10.9 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.7 Warrants issued to Berry-Shino Securities, Inc. (Incorporated by reference to Exhibit 10.10 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.8 Warrants issued to Neidiger, Tucker, Bruner, Inc. (Incorporated by reference to Exhibit 10.11 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.9 Form of warrant issued in March 2001 for guaranteeing debt (Incorporated by reference to Exhibit 10.12 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.10 Form of warrant issued in April 2002 for guaranteeing debt (Incorporated by reference to Exhibit 10.13 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 7

10.11 Articles of Organization of Hy-Bon Rotary Compression, L.L.C., dated April 17, 2000 (Incorporated by reference to Exhibit 10.18 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.12 Regulations of Hy-Bon Rotary Compression, L.L.C. (Incorporated by reference to Exhibit 10.19 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.13 First Amended and Restated Loan Agreement between the Registrant and Western National Bank (Incorporated by reference to Exhibit 10.1 of the Registrant's Current Report on Form 8-K, dated March 27, 2003 and filed with the Securities and Exchange Commission on April 14, 2003) 10.14 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Alan Kurus (Incorporated by reference to Exhibit 10.25 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.15 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Wayne Vinson (Incorporated by reference to Exhibit 10.26 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.16 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Earl Wait (Incorporated by reference to Exhibit 10.27 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.17 Triple Net Lease Agreement, dated June 1, 2003, between NGE Leasing, Inc. and Steven J. & Katherina L. Winer (Incorporated by reference to Exhibit 10.17 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2003) 10.18 Lease Agreement, dated June 19, 2003, between NGE Leasing, Inc. and Wise Commercial Properties (Incorporated by reference to Exhibit 10.18 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2003) 10.19 Lease Agreement, dated March 1, 2004, between the Registrant and the City of Midland, Texas (Incorporated by reference to Exhibit 10.19 of the Registrant's Form 10-QSB for the fiscal quarter ended June 30, 2004) 8

10.20 Second Amended and Restated Loan Agreement, dated November 3, 2003, between the Registrant and Western National Bank (Incorporated by reference to Exhibit 10.20 of the Registrant's Form 10-QSB for the fiscal quarter ended June 30, 2004) 10.21 Securities Purchase Agreement, dated July 20, 2004, between the Registrant and CBarney Investments, Ltd. (Incorporated by reference to Exhibit 4.1 of the Registrant's Current Report on Form 8-K dated July 20, 2004 and filed with the Securities and Exchange Commission on July 27, 2004) 10.22 Stock Purchase Agreement, dated October 18, 2004, by and among the Registrant, Screw Compression Systems, Inc., Paul D. Hensley, Jim Hazlett and Tony Vohjesus (Incorporated by reference to Exhibit 4.1 of the Registrant's Current Report on Form 8-K dated October 18, 2004 and filed with the Securities and Exchange Commission on October 21, 2004) 10.23 Fourth Amended and Restated Loan Agreement (Incorporated by reference to Exhibit 10.1 of the Registrant's Current Report on Form 8-K, dated March 14, 2005 as filed with the Securities and Exchange Commission on March 18, 2005) 10.24 Employment Agreement between Paul D. Hensley and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 10.1 of the Registrants Form 8-K Report, dated January 3, 2005, as filed with the Securities and Exchange Commission on January 7, 2005) 10.25 Employment Agreement between William R. Larkin and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 10.25 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 10.26 Promissory Note, dated January 3, 2005, in the original principal amount of $2,100,000.00 made by Natural Gas Services Group, Inc. payable to Paul D. Hensley (Incorporated by reference to Exhibit 10.26 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 14.0 Code of Ethics (Incorporated by reference to Exhibit 14.0 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 9

21.0 Subsidiaries (Incorporated by reference to Exhibit 21.0 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 23.1 Consent of Hein & Associates LLP (Incorporated by reference to Exhibit 23.1 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) *31.1 Certification of Chief Executive Officer required by Section 302 of the Sarbanes-Oxley Act of 2002 *31.2 Certification of Chief Financial Officer required by Section 302 of the Sarbanes-Oxley Act of 2002 *32.1 Certification required by Section 906 of the Sarbanes-Oxley Act of 2002 *32.2 Certification required by Section 906 of the Sarbanes-Oxley Act of 2002 - ------------------- * Filed herewith. 10

SIGNATURES In accordance with Section 13 or 15(d) of the Exchange Act, the registrant caused this report to be signed on its behalf by the undersigned, thereunto duly authorized. Date: May 3, 2005 NATURAL GAS SERVICES GROUP, INC. By: /s/ Stephen C. Taylor -------------------------------- Stephen C. Taylor, President and Principal Executive Officer By: /s/ Earl R.Wait -------------------------------- Earl R. Wait, Chief Financial Officer 11

EXHIBIT INDEX The following is a list of all exhibits filed as part of this Form 10-KSB/A: Exhibit No. Description - ----------- ----------- 2.1 Purchase and Sale Agreement by and between Hy-Bon Engineering Company, Inc. and NGE Leasing, Inc. (Incorporated by reference to Exhibit 2.1 of the Registrant's Current Report on Form 8-K dated February 28, 2003 and filed with the Securities and Exchange Commission on March 6, 2003) 3.1 Articles of Incorporation, as amended (Incorporated by reference to Exhibit 3.1 of the 10QSB filed and dated November 10, 2004) 3.2 Bylaws (Incorporated by reference to Exhibit 3.4 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.1 Form of warrant certificate (Incorporated by reference to Exhibit 4.1 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.2 Form of warrant agent agreement (Incorporated by reference to Exhibit 4.2 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.3 Form of lock-up agreement (Incorporated by reference to Exhibit 4.3 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.4 Form of representative's option for the purchase of common stock (Incorporated by reference to Exhibit 4.4 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.5 Form of representative's option for the purchase of warrants (Incorporated by reference to Exhibit 4.5 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 4.6 Stockholders Agreement, dated January 3, 2005 among Paul D. Hensley, Tony Vohjesus, Jim Hazlett and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 4.3 of the Registrant's From 8-K Report, dated January 3, 2005, as filed with the Securities and Exchange Commission on January 7, 2005) Executive Compensation Plans and Arrangements (Exhibits 10.1, 10.24, 10.25 and 10.26)

10.1 1998 Stock Option Plan (Incorporated by reference to Exhibit 10.1 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.2 Asset Purchase Agreement, dated January 1, 2001, between the Registrant and Great Lakes Compression, Inc. (Incorporated by reference to Exhibit 10.2 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.3 Exhibits 3(c)(1), 3(c)(2), 3(c)(3), 3(c)(4), 13(d)(1), 13(d)(2) and 13(d)(3) to Asset Purchase Agreement, dated January 1, 2001, between the Registrant and Great Lakes Compression, Inc. (Incorporated by reference to Exhibit 10.14 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.4 Amendment to Guaranty Agreement between Natural Gas Services Group, Inc. and Dominion Michigan Production Services, Inc. (Incorporated by reference to Exhibit 10.3 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.5 Form of Series A 10% Subordinated Notes due December 31, 2006 (Incorporated by reference to Exhibit 10.8 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.6 Form of Five-Year Warrants to Purchase Common Stock (Incorporated by reference to Exhibit 10.9 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.7 Warrants issued to Berry-Shino Securities, Inc. (Incorporated by reference to Exhibit 10.10 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.8 Warrants issued to Neidiger, Tucker, Bruner, Inc. (Incorporated by reference to Exhibit 10.11 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.9 Form of warrant issued in March 2001 for guaranteeing debt (Incorporated by reference to Exhibit 10.12 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.10 Form of warrant issued in April 2002 for guaranteeing debt (Incorporated by reference to Exhibit 10.13 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.11 Articles of Organization of Hy-Bon Rotary Compression, L.L.C., dated April 17, 2000 (Incorporated by reference to Exhibit 10.18 of the Registrant's Registration Statement on Form SB-2, No. 333-88314)

10.12 Regulations of Hy-Bon Rotary Compression, L.L.C. (Incorporated by reference to Exhibit 10.19 of the Registrant's Registration Statement on Form SB-2, No. 333-88314) 10.13 First Amended and Restated Loan Agreement between the Registrant and Western National Bank (Incorporated by reference to Exhibit 10.1 of the Registrant's Current Report on Form 8-K, dated March 27, 2003 and filed with the Securities and Exchange Commission on April 14, 2003) 10.14 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Alan Kurus (Incorporated by reference to Exhibit 10.25 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.15 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Wayne Vinson (Incorporated by reference to Exhibit 10.26 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.16 Form of Termination of Employment Agreement Letter relating to the Employment Agreement of Earl Wait (Incorporated by reference to Exhibit 10.27 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2002) 10.17 Triple Net Lease Agreement, dated June 1, 2003, between NGE Leasing, Inc. and Steven J. & Katherina L. Winer (Incorporated by reference to Exhibit 10.17 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2003) 10.18 Lease Agreement, dated June 19, 2003, between NGE Leasing, Inc. and Wise Commercial Properties (Incorporated by reference to Exhibit 10.18 of the Registrant's Annual Report on Form 10-KSB for the fiscal year ended December 31, 2003) 10.19 Lease Agreement, dated March 1, 2004, between the Registrant and the City of Midland, Texas (Incorporated by reference to Exhibit 10.19 of the Registrant's Form 10-QSB for the fiscal quarter ended June 30, 2004) 10.20 Second Amended and Restated Loan Agreement, dated November 3, 2003, between the Registrant and Western National Bank (Incorporated by reference to Exhibit 10.20 of the Registrant's Form 10-QSB for the fiscal quarter ended June 30, 2004) 10.21 Securities Purchase Agreement, dated July 20, 2004, between the Registrant and CBarney Investments, Ltd. (Incorporated by reference to Exhibit 4.1 of the Registrant's Current Report on Form 8-K dated July 20, 2004 and filed with the Securities and Exchange Commission on July 27, 2004)

10.22 Stock Purchase Agreement, dated October 18, 2004, by and among the Registrant, Screw Compression Systems, Inc., Paul D. Hensley, Jim Hazlett and Tony Vohjesus (Incorporated by reference to Exhibit 4.1 of the Registrant's Current Report on Form 8-K dated October 18, 2004 and filed with the Securities and Exchange Commission on October 21, 2004) 10.23 Fourth Amended and Restated Loan Agreement (Incorporated by reference to Exhibit 10.1 of the Registrant's Current Report on Form 8-K, dated March 14, 2005 as filed with the Securities and Exchange Commission on March 18, 2005) 10.24 Employment Agreement between Paul D. Hensley and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 10.1 of the Registrants Form 8-K Report, dated January 3, 2005, as filed with the Securities and Exchange Commission on January 7, 2005) 10.25 Employment Agreement between William R. Larkin and Natural Gas Services Group, Inc. (Incorporated by reference to Exhibit 10.25 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 10.26 Promissory Note, dated January 3, 2005, in the original principal amount of $2,100,000.00 made by Natural Gas Services Group, Inc. payable to Paul D. Hensley (Incorporated by reference to Exhibit 10.26 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 14.0 Code of Ethics (Incorporated by reference to Exhibit 14.0 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 21.0 Subsidiaries (Incorporated by reference to Exhibit 21.0 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) 23.1 Consent of Hein & Associates LLP (Incorporated by reference to Exhibit 23.1 of the Registrant's Form 10-KSB for the fiscal year ended December 31, 2004, and filed with the Securities and Exchange Commission on March 30, 2005) *31.1 Certification of Chief Executive Officer required by Section 302 of the Sarbanes-Oxley Act of 2002

*31.2 Certification of Chief Financial Officer required by Section 302 of the Sarbanes-Oxley Act of 2002 *32.1 Certification required by Section 906 of the Sarbanes-Oxley Act of 2002 *32.2 Certification required by Section 906 of the Sarbanes-Oxley Act of 2002 - ------------------- * Filed herewith.

                                                                    Exhibit 31.1

                 Certification of Chief Executive Officer Under
                  Section 302 of the Sarbanes-Oxley Act of 2002

         I, Stephen C. Taylor,  Chief Executive  Officer of Natural Gas Services
Group, Inc., certify that:

         1. I have reviewed this Amendment No. 1 on Form 10-KSB/A of Natural Gas
Services Group, Inc;

         2. Based on my  knowledge,  this  report  does not  contain  any untrue
statement of a material fact or omit to state a material fact  necessary to make
the statements made, in light of the  circumstances  under which such statements
were made, not misleading with respect to the period covered by this report;

         3. Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial condition,  results of operations and cash flows of the small business
issuer as of, and for, the periods presented in this report;

         4. The small  business  issuer's  other  certifying  officers and I are
responsible for establishing and maintaining  disclosure controls and procedures
(as  defined  in  Exchange  Act Rules  13a-15(e)  and  15d-15(e))  for the small
business issuer and have:

                  (a)  Designed  such  disclosure  controls and  procedures,  or
caused  such  disclosure  controls  and  procedures  to be  designed  under  our
supervision,  to ensure that material information relating to the small business
issuer, including its consolidated  subsidiaries,  is made known to us by others
within those  entities,  particularly  during the period in which this report is
being prepared;

                  (b) Omitted;

                  (c) Evaluated the effectiveness of the small business issuer's
disclosure  controls and procedures and presented in this report our conclusions
about the effectiveness of the disclosure controls and procedures, as of the end
of the period covered by this report based on such evaluation; and

                  (d) Disclosed in this report any change in the small  business
issuer's  internal  control over financial  reporting  that occurred  during the
small business  issuer's most recent fiscal quarter (the small business issuer's
fourth  fiscal  quarter  in the case of an annual  report)  that has  materially
affected,  or is reasonably  likely to  materially  affect,  the small  business
issuer's internal control over financial reporting; and

         5. The small business issuer's other certifying officer(s) and I have
disclosed, based on our most recent evaluation of internal control over
financial reporting, to the small business issuer's auditors and the audit
committee of the small business issuer's board of directors (or persons
performing the equivalent functions):

                  (a) All significant  deficiencies  and material  weaknesses in
the design or operation of internal  control over financial  reporting which are
reasonably  likely to adversely  affect the small business  issuer's  ability to
record, process, summarize and report financial information; and

                  (b)  Any  fraud,  whether  or  not  material,   that  involves
management or other employees who have a significant  role in the small business
issuer's internal control over financial reporting.


Date:  May 3, 2005


                                                          /s/ Stephen C. Taylor
                                                         -----------------------
                                                         Stephen C. Taylor
                                                         Chief Executive Officer


                                                                    Exhibit 31.2

                 Certification of Chief Financial Officer Under
                  Section 302 of the Sarbanes-Oxley Act of 2002

         I, Earl R. Wait, Chief Financial Officer of Natural Gas Services Group,
Inc., certify that:

         1. I have reviewed this Amendment No. 1 on Form 10-KSB/A of Natural Gas
Services Group, Inc;

         2. Based on my  knowledge,  this  report  does not  contain  any untrue
statement of a material fact or omit to state a material fact  necessary to make
the statements made, in light of the  circumstances  under which such statements
were made, not misleading with respect to the period covered by this report;

         3. Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial condition,  results of operations and cash flows of the small business
issuer as of, and for, the periods presented in this report;

         4. The small  business  issuer's  other  certifying  officers and I are
responsible for establishing and maintaining  disclosure controls and procedures
(as  defined  in  Exchange  Act Rules  13a-15(e)  and  15d-15(e))  for the small
business issuer and have:

                  (a)  Designed  such  disclosure  controls and  procedures,  or
caused  such  disclosure  controls  and  procedures  to be  designed  under  our
supervision,  to ensure that material information relating to the small business
issuer, including its consolidated  subsidiaries,  is made known to us by others
within those  entities,  particularly  during the period in which this report is
being prepared;

                  (b) Omitted;

                  (c) Evaluated the effectiveness of the small business issuer's
disclosure  controls and procedures and presented in this report our conclusions
about the effectiveness of the disclosure controls and procedures, as of the end
of the period covered by this report based on such evaluation; and

                  (d) Disclosed in this report any change in the small  business
issuer's  internal  control over financial  reporting  that occurred  during the
small business  issuer's most recent fiscal quarter (the small business issuer's
fourth  fiscal  quarter  in the case of an annual  report)  that has  materially
affected,  or is reasonably  likely to  materially  affect,  the small  business
issuer's internal control over financial reporting; and

         5. The small business  issuer's other certifying  officer(s) and I have
disclosed,  based  on our  most  recent  evaluation  of  internal  control  over
financial  reporting,  to the small  business  issuer's  auditors  and the audit
committee  of the  small  business  issuer's  board  of  directors  (or  persons
performing the equivalent functions):

                  (a) All significant  deficiencies  and material  weaknesses in
the design or operation of internal  control over financial  reporting which are
reasonably  likely to adversely  affect the small business  issuer's  ability to
record, process, summarize and report financial information; and

                  (b)  Any  fraud,  whether  or  not  material,   that  involves
management or other employees who have a significant  role in the small business
issuer's internal control over financial reporting.


Date:  May 3, 2005

                                                          /s/ Earl R. Wait
                                                         -----------------------
                                                         Earl R. Wait
                                                         Chief Financial Officer

                                                                    Exhibit 32.1

                            CERTIFICATION PURSUANT TO
                               18 U.S.C. ss.1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002


         In  connection  with the Annual  Report of Natural Gas Services  Group,
Inc. (the  "Company") on Form 10-KSB/A for the year ended  December 31, 2004, as
filed with the Securities and Exchange Commission (the "Report"),  I, Stephen C.
Taylor,  President and Chief  Executive  Officer of the Company,  hereby certify
pursuant  to 18 U.S.C.  ss.1350,  as  adopted  pursuant  to  Section  906 of the
Sarbanes-Oxley Act of 2002, that:

1.       The Report fully  complies  with the  requirements  of Section 13(a) or
         15(d) of the Securities Exchange Act of 1934, as amended; and

2.       The  information  contained  in  the  Report  fairly  presents,  in all
         material respects, the financial condition and results of operations of
         the Company.


                                                          /s/ Stephen C. Taylor
                                                         -----------------------
                                                         Stephen C. Taylor
                                                         Chief Executive Officer


                                                         May 3, 2005



                                                                    Exhibit 32.2


                            CERTIFICATION PURSUANT TO
                               18 U.S.C. ss.1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002


         In  connection  with the Annual  Report of Natural Gas Services  Group,
Inc. (the  "Company") on Form 10-KSB/A for the year ended  December 31, 2004, as
filed with the  Securities  and  Exchange  Commission  on the date  hereof  (the
"Report"),  I, Earl R. Wait,  Chief  Financial  Officer of the  Company,  hereby
certify pursuant to 18 U.S.C. ss.1350, as adopted pursuant to Section 906 of the
Sarbanes-Oxley Act of 2002, that:

1.       The Report fully  complies  with the  requirements  of Section 13(a) or
         15(d) of the Securities Exchange Act of 1934, as amended; and

2.       The  information  contained  in  the  Report  fairly  presents,  in all
         material respects, the financial condition and results of operations of
         the Company.



                                                          /s/  Earl R. Wait
                                                         -----------------------
                                                         Earl R. Wait
                                                         Chief Financial Officer


                                                         May 3, 2005