SECURED PROMISSORY NOTE

$ 16,089.00                    Deer Park, Texas                     May 15, 2000


        FOR VALUE RECEIVED,  B. Tom Carter, Jr. ("Maker") hereby promises to pay
to the order of HydroChem Holding, Inc., a Delaware corporation  ("Payee"),  the
principal amount of Sixteen Thousand Eighty Nine and 00/100 Dollars ($16,089.00)
plus interest  thereon at the rate of Six and 40/100 percent (6.40%)  compounded
annually, and payable as provided herein in lawful money of the United States of
America,  at Deer Park,  Texas, or at such other place as Payee of this Note may
from time to time designate by written notice to Maker.

        The principal amount of this Note and all accrued interest thereon shall
be due and  payable  upon the  earlier  of May 14,  2006 or any  termination  of
Maker's employment with the Payee or Payee's wholly-owned subsidiary,  HydroChem
Industrial Services, Inc., but not earlier than May 14, 2003.

               Pursuant  to a Pledge  Agreement  of even date  herewith  between
Maker  and  Payee,  and as more  specifically  described  therein,  this Note is
secured by the pledge of certain shares of common stock of Payee which are owned
beneficially  and of record by Maker and which are being  acquired  by the Maker
pursuant to a stock option exercise simultaneously herewith.

        This Note may be prepaid in whole or in part without premium or penalty.

        If this Note is placed in the hands of an attorney for collection  after
default, or if all or any part of the indebtedness represented hereby is proved,
established or collected in any court or in any bankruptcy, receivership, debtor
relief, probate or other court proceedings,  then Maker agrees to pay reasonable
attorneys'  fees and collection  costs to Payee in addition to the principal and
interest payable hereunder.

        If default is made in the payment of the  principal  or  interest  under
this Note,  or if a default  occurs  under any other  instrument  evidencing  or
securing  payment  hereof,  then in any  one or more  such  events,  the  entire
principal balance and accrued interest owing hereon shall at once become due and
payable,  at the option of Payee.  Failure to  exercise  this  option  shall not
constitute  a waiver  of the  right  to  exercise  the same in the  event of any
subsequent default.

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        Maker,  signers,  sureties,  and endorsers of this Note severally  waive
notice of  acceleration  of maturity if such shall occur,  demand,  presentment,
notice of dishonor,  diligence in  collecting,  grace,  notice and protest,  and
agree to one or more  extensions  for any period or periods of time and  partial
payments, before or after maturity, without prejudice to Payee.

        THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS
OF THE STATE OF TEXAS.

                                     MAKER:

                                                   /s/ B. Tom Carter, Jr.
                                                   ----------------------
                                                   B. Tom Carter, Jr.
                                                   5956 Sherry Lane
                                                   Suite No. 930
                                                   Dallas, Texas  75225

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