EXHIBIT 10.3

                         AMENDMENT TO GUARANTY AGREEMENT

         THIS AMENDMENT (the "Amendment"), dated as of June 10, 2002, is made by
and between NATURAL GAS SERVICES GROUP, INC., a Colorado corporation (the
"Guarantor"), and DOMINION MICHIGAN PRODUCTION SERVICES, INC., a Michigan
corporation, formerly known as Great Lakes Compression, Inc. (the "Seller").

                                    RECITALS

         A. The Guarantor and the Seller are parties to a Limited Recourse
Guaranty Agreement dated as of March 21, 2001 (the "Guaranty Agreement").

         B. The Guarantor anticipates raising additional funds through an
initial public offering of shares of its common stock and warrants to purchase
shares of its common stock that is to be underwritten in part by
Neidiger/Tucker/Bruner, Inc. and that is anticipated to take place in 2002.

         C. The Guarantor has requested that certain amendments be made to the
Guaranty Agreement, which the Seller is willing to make pursuant to the terms
and conditions set forth herein.

         NOW, THEREFORE, in consideration of the premises and of the mutual
covenants and agreements herein contained, it is agreed as follows:

         1. Capitalized terms used in this Amendment which are defined in the
Guaranty Agreement shall have the same meanings as defined therein, unless
otherwise defined herein.

         2. Section 12 of the Guaranty Agreement is hereby amended and restated
in its entirety to read as follows:

         "Section 12. Initial Public Offering. Guarantor presently plans to
         raise additional funds through a public offering (the "IPO") and agrees
         that to the extent Buyer has not discharged its obligations under the
         Purchase Money Documents, the first $3,500,000 of the net proceeds of
         the IPO shall be applied to the unpaid balance of the Deferred Purchase
         Price plus any accrued interest."

         3. No Other Changes. Except as explicitly amended by this Amendment,
all of the terms and conditions of the Guaranty Agreement shall remain in full
force and effect.

         4. Applicable Law. This Amendment shall be governed by the laws of
Michigan.

         5. References. All references in the Guaranty Agreement to "this
Guaranty" shall be deemed to refer to the Guaranty Agreement as amended hereby
and as the same may be further amended, modified or supplemented from time to
time; and any and all references in the Asset




Purchase Agreement to the Guaranty Agreement shall be deemed to refer to the
Guaranty Agreement as amended hereby and as the same may be further amended,
modified or supplemented from time to time.

         6. Miscellaneous. This Amendment may be executed in any number of
counterparts, each of which when so executed and delivered shall be deemed an
original and all of which counterparts, taken together, shall constitute one and
the same instrument.

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed as of the date first written above.

NATURAL GAS SERVICES GROUP, INC.         DOMINION MICHIGAN PRODUCTION
                                         SERVICES, INC. formerly known as Great
                                         Lakes Compression, Inc.

By:    /s/ Wayne L. Vinson
   -----------------------------
Name:  WAYNE L. VINSON
     ---------------------------
Its:   President
    ----------------------------         By:    /s/ Fred G. Wood III
                                            -----------------------------------
                                         Name:  FRED G. WOOD III
                                               --------------------------------
                                         Its:   Senior Vice President
                                             ----------------------------------



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