SECURED PROMISSORY NOTE



$346,355.86                      Deer Park, Texas                April 30,  1999


         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 Three Hundred  Forty Six Thousand  Three Hundred Fifty Five
and 86/100 Dollars  ($346,355.86)  plus interest thereon at the rate of Five and
28/100 percent (5.28)%  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 April 30, 2005 or any  termination
of  Maker's  employment  with the  Payee  or  Payee's  wholly-owned  subsidiary,
HydroChem Industrial Services, Inc., but not earlier than April 30, 2002.

         Of the principal amount of this Note, Forty Three Thousand Five Hundred
Thirty  Nine and 00/100  Dollars  ($43,539.00)  is in full  payment  for certain
shares of the Payee's Class A Common Stock being acquired by the Maker as of the
date hereof  pursuant to the partial  exercise of two stock  options  each at an
exercise  price of One Dollar  ($1.00) per share.  The balance of the  principal
represents a substitution  and  replacement of Maker's  Secured  Promissory Note
(including  accrued  interest  thereon)  dated  April  9,  1998 to  Payee as the
assignee of HydroChem Industrial Services, Inc.

         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  include,  but are not limited to, the shares
being  acquired  by the  Maker  pursuant  to  the  aforementioned  stock  option
exercise.

         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




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shall not  constitute a waiver of the right to exercise the same in the event of
any subsequent default.

         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 #930
                                                       Dallas, Texas  75225