GUARANTY & AGREEMENT


Guaranty  and  Agreement  ("Guaranty") between the Guarantor named below and RZB
FINANCE  LLC  (together  with  its  successors  and assigns, "RZB"), dated as of
September  15,  2004.

1.   SPECIAL  TERMS

     The  following  terms  and  provisions  shall  apply  to this Guaranty; the
     meaning of any term in this or other sections of this Guaranty expressed in
     the  singular  shall apply, mutatis mutandis, to the same term expressed in
                                 ------- --------
     the  plural  and  vice  versa.
                       ----  -----

     BORROWER:   Penn  Octane  Corporation,  a  Delaware  corporation

     GUARANTOR  :Rio  Vista Energy Partners L.P., a Delaware limited partnership

     GUARANTEED  OBLIGATIONS:  All  indebtedness,  liabilites and obligations of
     the  Borrower  to  RZB now existing or hereafter arising including, but not
     limited  to,  those  arising  under  the following documents (including any
     modifications  thereof  or  substitutions  therefor, the "Loan Documents"):

     DOCUMENTS:  Amended  and  Restated  Line  Letter, dated as of September 15,
     2004,  (the  "Line  Letter"), General Security Agreement, dated October 18,
     2002,  Continuing  Agreement for Letters of Credit, dated October 14, 1997,
     Promissory  Note,  dated  September  15,  2004

2.   GUARANTY

     2.1  Continuing  Guaranty  of Payment.  In consideration of RZB agreeing to
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     the Loan Documents and/or extending or continuing credit to the Borrower in
     connection  therewith,  the  Guarantor  irrevocably,  absolutely  and
     unconditionally  guarantees  to  RZB the payment when due of all Guaranteed
     Obligations,  together  with  interest  thereon  and  other charges related
     thereto. For purposes of this Guaranty, the Guaranteed Obligations shall be
     due  on  the  earliest  of:

     2.a. the  due  date  thereof  (by  acceleration  or  otherwise),

     2.b. with  respect  to  any  obligation due on demand, upon demand therefor
          made  by  RZB  upon  the  Borrower  or  the  Guarantor,

     2.c. the  giving  of  notice  by  RZB  to  the Borrower or Guarantor of the
          occurrence  of  any  default by the Guarantor hereunder (including any
          material  misrepresentation  by  the Guarantor herein or in connection
          herewith),

     2.d. the  occurrence  of  a  material adverse change in the Borrower or the
          Guarantor,

     2.e. the  Borrower  shall disaffirm or disavow any of its obligations under
          the  Loan  Documents  or  the  Guarantor  shall  disaffirm  any of its
          obligations  hereunder,

     2.f. the  Borrower or the Guarantor shall admit in writing its inability to
          pay  its  debts  as  they  become  due,

     2.g. any  indebtedness  (direct  or  contingent)  for borrowed money of the
          Borrower  or  the  Guarantor  shall  not  be paid as and when the same
          becomes  due  and  payable,  including any applicable grace period, or

     2.h. the  commencement  of any bankruptcy, insolvency or similar proceeding
          by  or  against  the  Borrower  or  the  Guarantor.

     This  is  a  guaranty  of payment rather than of collection; this is also a
     continuing  guaranty and all liabilities to which this Guaranty applies, or
     may apply, under the terms hereof shall be presumed to have been created in
     reliance  hereon.

     2.2  Nature  of  Obligations.  The  obligations  of  the  Guarantor to make
          -----------------------
     payments  to  RZB  hereunder are direct and primary obligations which shall
     not  be  discharged  for any reason until RZB has been indefeasibly paid in
     full.  Without limiting the generality of the foregoing, the obligations of
     the  Guarantor  hereunder  shall  remain  in  force  irrespective  of:

     2.a. any  invalidity,  illegality or unenforceability of, or any defect in,
          any  of  the  Loan  Documents or Guaranteed Obligations or any defense
          which  the  Borrower  may  have  with  respect  thereto,

     2.b. the existence or absence of any legal action to enforce the Guaranteed
          Obligations  or  the  Loan  Documents  or  any  security therefor, the
          issuance  of  any  judgment  therefor  or  the  execution  of any such
          judgment,  or

     2.c. any  other  circumstance  which  might  otherwise constitute a defense
          available  to  or  discharge  of,  a  guarantor or surety of any type.

     This guaranty is several and independent of, and may be enforced regardless
     of,  any  other  obligation  (direct or contingent) of the Guarantor or any
     other  "Person"  (such  term to include any person or legal or governmental
     entity,  association,  agency  or  instrumentality)  with  respect  to  the
     Guaranteed  Obligations.

     2.3  Payments.  All  payments  by  the Guarantor hereunder shall be made to
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     RZB  without  set-off,  recoupment, deduction or counterclaim at its office
     set  forth below (or as RZB may otherwise direct) in lawful currency of the
     United  States  of  America  and  in  immediately  available funds. Without
     limiting the foregoing, all payments hereunder shall be made free and clear
     of,  and  without deduction for, any present or future withholding or other
     taxes  or  duties,  including  stamp duties, or other charges of any nature
     imposed on such payments by or on behalf of any government or any political
     subdivision  or  agency  thereof  or  therein. If any such taxes, duties or
     charges  are  so  levied


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<PAGE>
     or imposed on any such payment, the Guarantor will make additional payments
     in such amounts as may be necessary so that the net amount received by RZB,
     after  deduction  for  or  on account of all such taxes, duties or charges,
     will  be  equal  to  the  amount  provided  for herein. The Guarantor shall
     furnish  promptly  to  RZB  official receipts evidencing the payment of any
     such  taxes,  duties  or  charges  paid  by  the  Guarantor.

3.   SPECIAL  AGREEMENTS  OF  GUARANTOR

     3.1  Subordination.  Subject to the next following sentence of this Section
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     3.1:

     3.a. all  claims of the Guarantor against the Borrower shall be subject and
          subordinate  to the prior payment to RZB of all Guaranteed Obligations
          and  all  obligations  of  the  Guarantor  hereunder,  and

     3.b. the Guarantor shall not be entitled to receive any payment or exercise
          any  set-off  in respect of any such claim and, to the extent any such
          payment  is  received  (whether  directly,  by  way  of  dividend  in
          bankruptcy,  set-off  or  otherwise),  the  Guarantor  will  forthwith
          deliver  the  same (or the value thereof) to RZB in precisely the form
          received  (except  for endorsement or assignment where necessary), for
          application to the Guaranteed Obligations and, until so delivered, the
          same  shall  be  held  in  trust  as  the  property  of  RZB.

     Notwithstanding the foregoing, except as provided in Section 3.2, until the
     occurrence  of  any default or event of default under the Loan Documents or
     this  Guaranty,  or  any  demand  for  payment  of  any  of  the Guaranteed
     Obligations, the Guarantor may receive and retain payment in respect of any
     obligation  owed  to it by the Borrower. If the Guarantor fails to make any
     necessary  endorsement  or assignment of any instrument of payment to which
     RZB  is  entitled,  RZB  and  any  of  its officers or employees are hereby
     irrevocably  authorized  to  make  the  same  on  behalf  of the Guarantor.

     3.2  No  Subrogation.  The Guarantor hereby waives any right of subrogation
          ---------------
     that  it  may have with respect to any payment that it may have made to RZB
     hereunder.

     3.3  No  Contribution.  The Guarantor agrees that it shall have no legal or
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     equitable  right  or  claim  (by  way  of  indemnification, contribution or
     otherwise)  against  any  subsidiary or affiliate of RZB which has issued a
     guaranty  to  RZB  in  respect  of  the  Guaranteed  Obligations.

     3.4  Waivers.  Except to the extent required by law which cannot be waived,
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     the  Guarantor  waives  notice of acceptance of this Guaranty and notice of
     any  liability  to  which  it may apply, and waives diligence, presentment,
     demand  for  payment, protest, notice of dishonor or nonpayment of any such
     liabilities,  suit  or  taking  other  action  or  making any demand by RZB
     against,  and  any other notice to, any party liable thereon (including the
     Guarantor).  The Guarantor agrees that RZB may at any time and from time to
     time,  upon  or  without  any  terms or conditions and in whole or in part:

     3.a. change the manner, place or terms of, and/or change or extend the time
          of payments of, renew or alter, any of the Guaranteed Obligations, any
          security therefor, or any liability incurred directly or indirectly in
          respect  thereof,  and  this  Guaranty  shall  apply to the Guaranteed
          Obligations  so  changed,  extended,  renewed  or  altered,

     3.b. fail  to  record,  perfect  or  protect,  or  sell, exchange, release,
          surrender,  realize  upon  or otherwise deal with in any manner and in
          any  order,  any property or Person whatsoever at any time securing or
          guaranteeing  the Guaranteed Obligations or any liabilities (including
          any  of  those  hereunder)  incurred directly or indirectly in respect
          thereof  or  hereof,  and/or  any  offset  thereagainst,

     3.c. exercise or refrain from exercising any rights against the Borrower or
          any other Person (including any guarantor) or otherwise act or refrain
          from  acting,

     3.d. settle,  release  or compromise any of the Guaranteed Obligations, any
          security  therefor or any liability (including any of those hereunder)
          incurred  directly or indirectly in respect thereof or hereof, and may
          subordinate  the  payment  of  all  or  any  part  of  the  Guaranteed
          Obligations to the payment of any other liability (whether due or not)
          of  the  Borrower  to  the  creditors of the Borrower (including RZB),

     3.e. apply  any  sums  by  whomsoever  paid  or  howsoever  realized to any
          liability  or  liabilities  of  the Borrower to RZB regardless of what
          liability  or liabilities of the Borrower remain unpaid, provided that
          payments  by  the Guarantor pursuant to this Guaranty shall be applied
          to  its obligations hereunder, but in such order as RZB may determine,

     3.f. consent  to  or waive any breach of or act, omission or default under,
          or  modify  or  amend  any  provision  of,  the Loan Documents, and/or

     3.g. increase  the  amount  of indebtedness of the Borrower to RZB, whether
          under  the  Loan  Documents  or  otherwise.

     It  is  understood and agreed that RZB may take any such action without the
     consent  of,  or notice to, the Guarantor, without incurring responsibility
     to the Guarantor, and without impairing or releasing the obligations of the
     Guarantor  hereunder.

     3.5  Amounts  Reclaimed.  If  claim  is  made  upon  RZB  for  repayment or
          ------------------
     recovery  of  any  amount  received  on  account  of  any of the Guaranteed
     Obligations  and  RZB  repays  all  or  part  of  said amount by reason of:

     3.a. any  judgment,  decree  or  order of any court, administrative body or
          trustee  in  bankruptcy  (or  other  Person  acting  on  behalf of the
          Borrower  or  its  estate)  or,

     3.b. any  settlement  or  compromise of any such claim effected by RZB with
          any  such  claimant  (including  the  Borrower),  or

     c.   any  other  reason,

     then,  and  in any such event, the Guarantor agrees that any such repayment
     (by reason of any such judgment, decree, order, settlement or compromise or
     otherwise)  shall  be  binding  upon  the  Guarantor,  notwithstanding  any
     cancellation  of  the  Loan  Documents  or this Guaranty, and the Guarantor
     shall  be  and  remain  liable to RZB hereunder for the amount so repaid or
     recovered  to  the same extent as if such amount had never been received by
     RZB.


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<PAGE>
     3.6  Interest.  If  the  Guarantor  fails  to  pay  when due any obligation
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     hereunder, then, to the extent permitted by law, such obligation shall bear
     interest,  payable  on  demand,  from  the due date thereof until paid at a
     fluctuating  rate  per annum equal to 2% in excess of the Base Lending Rate
     (as  hereinafter  defined); provided that no such additional interest shall
     be  payable  in  respect  of  any  such  obligation  on  which  interest is
     simultaneously  accruing  pursuant  to  the  Loan Documents. The term "Base
                                                                            ----
     Lending  Rate"  means, for any day, the higher of (i) the rate announced by
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     The  Chase  Manhattan  Bank (the "Bank") from time to time at its principal
     office in New York, New York as its prime rate for domestic (United States)
     commercial  loans in effect on such day and (ii) the Federal Funds Rate (as
     hereinafter  defined)  in  effect on such day plus 1/2%. (Such Base Lending
     Rate  is not necessarily intended to be the lowest rate of interest charged
     by  the  Bank  in connection with extensions of credit.) Each change in the
     Base  Lending  Rate  shall result in a corresponding change in the interest
     rate  and  such  change  shall  be  effective on the effective date of such
     change  in  the  Base  Lending  Rate.

     The  term  "Federal  Funds  Rate" means, for any day, the overnight federal
                 --------------------
     funds  rate in New York City, as published for such day (or, if such day is
     not  a New York Business Day, for the next preceding New York Business Day)
     in  the  Federal  Reserve  Statistical  Release H.15 (519) or any successor
     publication, or if such rate is not so published for any day which is a New
     York  Business Day, the average of the quotations for such day on overnight
     federal  funds  transactions  in  New York City received by RZB or the Bank
     from  three federal funds brokers of recognized standing selected by RZB or
     the  Bank.

     3.7  The  Borrower,  etc.  The Guarantor will not cease to own (directly or
          -------------------
     indirectly),  free and clear of all liens and encumbrances, the interest in
     the  Borrower or any other Loan Party (as defined in the Line Letter) which
     it  presently  owns  (if  any) and will not agree to sell or subject to any
     lien,  encumbrance or any other security device such interest at any future
     time,  until  the  Loan  Documents  are  terminated  and  all  Guaranteed
     Obligations  and  all  obligations  of  the Guarantor hereunder are paid in
     full.

     3.8  Information.  The  Guarantor  will  promptly  furnish  to  RZB  such
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     information  regarding  its  and  the  Borrower's  business,  affairs  and
     financial  condition  as  RZB  may  from  time  to time reasonably request.

     3.9  Secured  Obligations.  The  Guarantor shall not grant, create, assume,
          --------------------
     incur or suffer to exist a security interest in or lien or encumbrance upon
     any  of  its  property  for  the purpose of securing the obligations of the
     Borrower  and/or  itself  to any other Person unless such Person shall have
     entered  into  an  intercreditor  agreement  with RZB in form and substance
     satisfactory  to  RZB.

4.   REPRESENTATIONS,  WARRANTIES  AND  AGREEMENTS

     In  order  to  induce  RZB  to  agree  to  the Loan Documents, to extend or
     continue  the  credit  provided  thereby  and  to accept this Guaranty, the
     Guarantor  makes  the  following representations, warranties and agreements
     which  shall  survive  the  execution  and  delivery  of  this  Guaranty:

     4.1  Organizational  Status  and  Power.  The  Guarantor:
          ----------------------------------

     4.a. is, and will continue to be, duly organized and validly existing under
          the  law  of  the  jurisdiction  of  its organization, as indicated in
          Section  1,  and

     4.b. has  the  power  to  own  its  assets,  to conduct its business as now
          conducted  and  to  enter  into  and  perform  the  provisions of this
          Guaranty.

     4.2  Legality.  The  entering into and performance by the Guarantor of this
          --------
     Guaranty:

     4.a. have  been  duly authorized by all necessary corporate and stockholder
          action  or  other action required by its organizational documents, and

     4.b. do  not  contravene any existing law or any legal order applicable to,
          or  license  or  permit granted to, the Guarantor, or any agreement or
          instrument  to which the Guarantor is a party or to which it or any of
          its  assets  is  subject  or  any  provision  of  the  Guarantor's
          organizational  documents.

     This  Guaranty is the legal, valid and binding obligation of the Guarantor,
     enforceable  in  accordance  with  its  terms.

     4.3  Governmental  Approvals.  Neither  any  action  by  or  with  any
          -----------------------
     governmental  or  public  body or authority (including, without limitation,
     any  exchange  control  or monetary authority), or any subdivision thereof,
     nor  any  other legal formality is required in connection with the entering
     into,  performance  or  enforcement  of  this  Guaranty  (collectively,
     "Governmental  Approvals"),  except  such as has been obtained or taken and
     with  respect  to  which  a copy or other satisfactory evidence thereof has
     been  furnished  to  RZB.  The  Guarantor  will  maintain  all  requisite
     Governmental  Approvals  until  the  Loan  Documents are terminated and the
     Guaranteed  Obligations  and  all  of its obligations hereunder are paid in
     full.

     4.4  Financial Condition.  The most recent year-end financial statements of
          -------------------
     the  Guarantor furnished to RZB prior to the date hereof have been prepared
     in  accordance  with  generally accepted accounting principles consistently
     applied  and  fairly  present  the  financial  condition and the results of
     operations  of  the Guarantor as at the end of and for the reporting period
     covered  thereby.  There  are  no  material  liabilities  or  any  material
     unrealized or anticipated losses from unfavorable commitments which are not
     disclosed  in such financial statements. There has been no material adverse
     change  in the operations, business or financial condition of the Guarantor
     from  that  set  forth in such financial statements; and there are no legal
     proceedings  pending  or,  to  the  knowledge  of the Guarantor threatened,
     against  or  affecting  the  Guarantor  which might (individually or in the
     aggregate)  result  in  such  a  material  adverse  change.

     4.5  Investment  Company  Act.  The  Guarantor  is not required to register
          ------------------------
     under  the  Investment Company Act of 1940, as amended (the "Act"), and the
     entering  into  of this Guaranty and the performance thereof do not violate
     any  provision  of  the  Act.

     On  each  anniversary  of  the  date of this Agreement, the Guarantor shall
     deliver  to  RZB  a  certificate  of an authorized officer of the Guarantor
     wherein  the  Guarantor  shall  reaffirm  to  RZB  the continuing truth and
     validity  of  the representations and warranties set forth in the foregoing
     Section  4.  The  failure  of the Guarantor to deliver and/or RZB to demand
     such  delivery  of  the  foregoing  certificate  shall  in no way affect or
     invalidate  the continuing nature of the representations and warranties set
     forth  in  the  foregoing  Section  4.


                                      -3-
<PAGE>
5.   MISCELLANEOUS

     5.1  Payment  of  Expenses.  The  Guarantor agrees to pay all out-of-pocket
          ---------------------
     costs and expenses of RZB arising in connection with its administration and
     enforcement  of,  or  preservation  of  its  rights  under,  this  Guaranty
     (including, without limitation, the reasonable fees and expenses of counsel
     for  RZB),  and  all stamp taxes (including interest and penalties, if any)
     which  may be payable in respect of this Guaranty or of any modification of
     this  Guaranty.

     5.2  Modification.  This  Guaranty  may  be  modified  or waived only by an
          ------------
     instrument  in  writing signed by the party against whom enforcement of the
     modification  or  waiver  is  sought.

     5.3  THIS  GUARANTY AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES HEREUNDER
     SHALL  BE  GOVERNED  BY,  AND  CONSTRUED IN ACCORDANCE WITH, THE LAW OF THE
     STATE  OF  NEW  YORK  (WITHOUT  REGARD  TO PRINCIPLES OF CONFLICTS OF LAW).

     5.4  Notices.  Communications  given  to  any party in connection with this
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     Guaranty  shall  be  in  English and in writing and shall be effective when
     delivered  at  its  address set forth herein, as the same may be changed by
     written  notice  to  the  other party. Written communications may be in any
     form  of  writing  howsoever  transmitted.

     5.5  Waiver.  RZB's  rights,  powers,  privileges  and  remedies under this
          ------
     Guaranty  or  applicable law are cumulative and not exclusive and shall not
     be  waived,  precluded  or  limited by any failure or delay in the exercise
     thereof or by RZB's exercise, or partial exercise, of any thereof or by any
     course  of dealing between the Guarantor and RZB. No notice to or demand on
     the  Guarantor  in  any  case  shall  entitle the Guarantor to any other or
     further  notice or demand in similar or other circumstances or constitute a
     waiver  of  the  right  of  RZB  to  any  other  or  further  action in any
     circumstance  without  notice  or  demand.

     5.6  Descriptive  Headings.  The descriptive headings used in this Guaranty
          ---------------------
     are  for  convenience only and shall not be deemed to affect the meaning or
     construction  of  any  provision  hereof.

     5.7  Benefit  of  Guaranty.  This  Guaranty  shall  be  binding  upon  the
          ---------------------
     Guarantor and its successors and assigns and shall inure to the benefit of,
     and  be  enforceable  by,  RZB  and  its  successors  and  assigns  and, in
     particular, any holder or assignee from time to time of the Loan Documents;
     provided that the Guarantor may not assign any of its rights or obligations
     hereunder  without  the  prior  written  consent  of  RZB.

     5.8  Set-Off.  RZB is authorized at any time and from time to time, without
          -------
     notice  to  the  Guarantor  or  to  any other Person, any such notice being
     hereby expressly waived, to set off and apply any and all deposits (general
     or  special) and any other indebtedness at any time held or owing by RZB to
     or for the credit or the account of the Guarantor against and on account of
     the  obligations  of  the  Guarantor  under  this Guaranty, irrespective of
     whether  or  not RZB shall have made any demand hereunder or any demand for
     payment  of  any  Guaranteed  Obligation  and  although  said  obligations,
     liabilities  or  claims,  or any of them, shall be contingent or unmatured,
     and  the  Guarantor  hereby  grants  to RZB a security interest in all such
     deposits  and  indebtedness.

     5.9  Jurisdiction  and  Immunity.  The Guarantor represents and agrees that
          ---------------------------
     it  is  not entitled to, and to the extent it hereafter becomes so entitled
     hereby waives, any immunity, sovereign or otherwise, with respect to itself
     and  its  property  from jurisdiction, service, attachment (both before and
     after  judgment)  and  execution in legal proceedings wherever commenced to
     enforce  or  collect  upon  this  Guaranty.

     5.10  Survival.  The provisions of Sections 3.5, 5.1 and 5.11 shall survive
           --------
     the  termination  and cancellation of this Guaranty and, after cancellation
     and  return  to  the  Guarantor,  a  photocopy  hereof  may be submitted as
     evidence  of  such surviving obligations. Nothing herein shall preclude RZB
     from  establishing  such  obligations  by  other  means.

     5.11 WAIVER OF JURY TRIAL.  EACH OF THE GUARANTOR AND RZB HEREBY KNOWINGLY,
          --------------------
     VOLUNTARY  AND  INTENTIONALLY  WAIVES  ANY  AND ALL RIGHTS IT MAY HAVE TO A
     TRIAL  BY  JURY  IN  RESPECT OF ANY LITIGATION BASED ON, OR ARISING OUT OF,
     UNDER,  OR  IN  CONNECTION  WITH  THIS  GUARANTY  OR  ANY OTHER DOCUMENT OR
     AGREEMENT EXECUTED IN CONNECTION HEREWITH, OR ANY COURSE OF CONDUCT, COURSE
     OF  DEALING,  STATEMENTS  (WHETHER  VERBAL  OR  WRITTEN), OR ACTIONS OF THE
     GUARANTOR, THE BORROWER OR RZB. THIS PROVISION IS A MATERIAL INDUCEMENT FOR
     RZB'S  EXTENDING TO THE BORROWER THE CREDIT FACILITY TO WHICH THIS GUARANTY
     RELATES.

     5.12  Consent  to Jurisdiction.  The Guarantor hereby agrees that ANY LEGAL
           ------------------------
     ACTION OR PROCEEDING AGAINST THE GUARANTOR WITH RESPECT TO THIS GUARANTY OR
     ANY  LOAN DOCUMENT MAY BE BROUGHT IN THE COURTS OF THE STATE OF NEW YORK IN
     THE  CITY  OF  NEW YORK OR OF THE UNITED STATES OF AMERICA FOR THE SOUTHERN
     DISTRICT  OF  NEW  YORK,  as  RZB may elect, and, by execution and delivery
     hereof, the Guarantor accepts and consents to, for itself and in respect of
     its  property,  generally  and  unconditionally,  the  jurisdiction  of the
     aforesaid  courts  and  agrees  that  such jurisdiction shall be exclusive,
     unless  waived  by RZB in writing, with respect to any action or proceeding
     brought  by  it  against  RZB  and  any question relating to usury. Nothing
     herein  shall  limit  the  right  of  RZB  to bring proceedings against the
     Guarantor  in  the courts of any other jurisdiction. Service of process out
     of  any  such courts may be made by mailing copies thereof by registered or
     certified  mail,  postage  prepaid,  to  the  Guarantor  at its address for
     notices  as  specified  herein and will become effective 30 days after such
     mailing.  The  Guarantor  agrees  that  Sections  5-1401  and 5-1402 of the
     General  Obligations  Law  of  the  State  of  New York shall apply to this
     Guaranty  and  the  Loan  Documents and, to the maximum extent permitted by
     law,  waives  any  right  to  any  defense  of, or to dismiss any action or
     proceeding brought before said court on the basis of, forum non conveniens.
                                                           ----- --- ----------


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<PAGE>
IN  WITNESS  WHEREOF,  the Guarantor has caused this Guaranty to be executed and
delivered  by its duly authorized officer(s) as of the date first above written.

NAME OF GUARANTOR:

RIO VISTA ENERGY PARTNERS L.P.
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By:  RIO VISTA GP LLC, general partner

     By: /s/ Richard Shore, Jr.
         -----------------------------
     Name: Richard  Shore,  Jr.
           ---------------------------
     Title: President
            --------------------------

ADDRESS OF GUARANTOR:

Rio Vista Energy Partners L.P.
820 Gessner Road
Houston, TX 77024


ACCEPTED AS OF THE DATE FIRST ABOVE WRITTEN:

RZB FINANCE LLC



By:  /s/ Pearl Geffers
     ---------------------------------
     Pearl Geffers

Title: First Vice President
        ------------------------------


By:  /s/  Griselda Alvizo
     ---------------------------------
     Griselda Alvizo

Title: Vice President
       -------------------------------


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