<PAGE>

                                                                    EXHIBIT 10.1

                CONTRIBUTION, CONVEYANCE AND ASSUMPTION AGREEMENT

         This Contribution, Conveyance and Assumption Agreement (this
"Agreement") dated effective as of ______________, 2003, is entered into by and
among Penn Octane Corporation, a Delaware corporation ("POCC"), Rio Vista GP
LLC, a Delaware limited liability company (the "GP"), Rio Vista Energy Partners
L.P., a Delaware limited partnership (the "MLP"), Rio Vista Operating GP LLC, a
Delaware limited liability company (the "Operating GP"), and Rio Vista Operating
Partnership L.P., a Delaware limited partnership (the "Operating Partnership").

                                    RECITALS

         WHEREAS, prior to the date hereof, POCC formed the GP, as a
wholly-owned direct subsidiary, and purchased for $1,000.00 all of the limited
liability company interests in the GP;


         WHEREAS, the GP and POCC formed the MLP, with the GP purchasing a 2%
general partner interest for $20 and POCC purchasing common units representing a
98% limited partner interest for $980.00;


         WHEREAS, the MLP formed the Operating GP and purchased all of the
limited liability company interests in the Operating GP for $1,000.00;

         WHEREAS, POCC and the Operating GP formed the Operating Partnership,
with the Operating GP purchasing a 0.10% general partner interest for $1.00 and
POCC purchasing a 99.9% limited partner interest for $999.00;

         WHEREAS, each of the following transactions shall occur as of 12:01
A.M. Eastern Time on ________, 2003 (the "Contribution Effective Time"):

         1. POCC will contribute all of the outstanding capital stock of its
subsidiaries (the "Subsidiary Interests") set forth on Exhibit A (the
"Subsidiaries") hereto to the Operating Partnership as a capital contribution;

         2. POCC will contribute to the Operating Partnership the assets set
forth in the Conveyance Agreement described in Section 1.2 below (the "LPG
Assets") as an additional capital contribution; and

         3. The GP will convey $1,000 to the MLP in exchange for the issuance of
incentive distribution rights to the GP;

         WHEREAS, each of the following transactions shall occur as of 12:02
A.M. Eastern Time on ___________, 2003 (the "Closing Day Effective Time"):

         1. POCC will contribute all of its limited partner interest in the
Operating Partnership to the MLP as an additional capital contribution.

         NOW, THEREFORE, in consideration of their mutual undertakings and
agreements hereunder, the parties to this Agreement undertake and agree as
follows:

                                    ARTICLE I

         CONVERSIONS, CONTRIBUTIONS AND DISTRIBUTIONS OF VARIOUS ASSETS

         SECTION 1.1 CONTRIBUTION OF THE SUBSIDIARY INTERESTS BY POCC TO THE
OPERATING PARTNERSHIP. At the Contribution Effective Time, POCC hereby grants,
contributes, transfers, assigns and conveys to the Operating Partnership, its
successors and assigns, all right, title and interest in and to the Subsidiary
Interests as a capital contribution and the Operating Partnership hereby accepts
the Subsidiary Interests.

<PAGE>

         SECTION 1.2 CONTRIBUTION OF LPG ASSETS BY POCC TO THE OPERATING
PARTNERSHIP. At the Contribution Effective Time, POCC hereby grants,
contributes, transfers, assigns and conveys to the Operating Partnership, its
successors and assigns, all right, title and interest in and to the LPG Assets,
and the Operating Partnership hereby accepts the LPG Assets. In order to give
full effect to the foregoing grant, contribution, transfer, assignment and
conveyance, POCC, as grantor, and the Operating Partnership, as grantee, shall
execute a Conveyance Agreement in the form attached hereto as Exhibit B together
with such other special warranty deeds, conveyances or other documents required
to transfer the LPG Assets in the jurisdictions in which they are located.

         SECTION 1.3 CONTRIBUTION OF OPERATING PARTNERSHIP INTEREST BY POCC TO
THE MLP. At the Closing Day Effective Time, POCC hereby grants, contributes,
transfers, assigns and conveys to the MLP, its successors and assigns, all
right, title and interest of POCC in and to the 99.9% limited partner interest
in the Operating Partnership as an additional capital contribution to the MLP
and the Operating Partnership hereby accepts such limited partner interest as an
additional capital contribution to the Operating Partnership.

         SECTION 1.4 CONVEYANCE BY THE GP TO THE MLP. At the Closing Day
Effective Time, the GP hereby conveys to the MLP $1,000.00 in exchange for all
of the incentive distribution rights under the First Amended and Restated
Agreement of Limited Partnership of the MLP.

                                   ARTICLE II

                 RECORDATION OF EVIDENCE OF OWNERSHIP OF ASSETS

         SECTION 2.1. In connection with the conveyances that are referred to in
Article I to this Agreement, the parties to this Agreement acknowledge that
certain jurisdictions in which the assets of the applicable parties to such
conveyances are located may require that documents be recorded by such parties
resulting from such conveyances in order to evidence title to the assets owned
by such parties. All such documents shall evidence such new ownership and are
not intended to modify, and shall not modify, any of the terms, covenants and
conditions herein set forth.

                                   ARTICLE III

                        ASSUMPTION OF CERTAIN LIABILITIES

         SECTION 3.1 ASSUMPTION OF LIABILITIES AND OBLIGATIONS BY THE OPERATING
PARTNERSHIP AND THE MLP. In connection with the contributions of the LPG Assets
and the Subsidiary Interests to the Operating Partnership, the Operating
Partnership hereby assumes and agrees to duly and timely pay, perform and
discharge all obligations and liabilities associated with the Contributed
Assets, that arise from and after the date of this Agreement, to the full extent
that either of the Subsidiaries or POCC would have been obligated to pay,
perform and discharge such obligations and liabilities in the future, were it
not for the execution and delivery of this Agreement; provided, however, that
said assumption and agreement to duly and timely pay, perform and discharge such
obligations and liabilities shall not increase the obligation of the Operating
Partnership with respect to such obligations and liabilities beyond that of POCC
as to the LPG Assets, or the Subsidiaries as to the assets acquired by the
Operating Partnership in such interest conveyed by any of POCC or the
Subsidiaries. For purposes of this Agreement, the term "Contributed Assets"
shall mean, collectively, the LPG Assets and the Subsidiary Interests.

                                       2
<PAGE>

                                   ARTICLE IV

                                  TITLE MATTERS

SECTION 4.1       DISCLAIMER OF WARRANTIES; SUBROGATION.

(a)      (i) NOTWITHSTANDING ANYTHING TO THE CONTRARY CONTAINED IN THIS
         AGREEMENT, THE MLP AND THE OPERATING PARTNERSHIP ACKNOWLEDGE AND AGREE
         THAT POCC AND THE SUBSIDIARIES HAVE NOT MADE, DO NOT MAKE, AND
         SPECIFICALLY NEGATE AND DISCLAIM, ANY REPRESENTATIONS, WARRANTIES,
         PROMISES, COVENANTS, AGREEMENTS OR GUARANTIES OF ANY KIND OR CHARACTER
         WHATSOEVER, WHETHER EXPRESS, IMPLIED OR STATUTORY, ORAL OR WRITTEN,
         PAST OR PRESENT (ALL OF WHICH ARE EXPRESSLY DISCLAIMED BY POCC AND THE
         SUBSIDIARIES) REGARDING (1) THE TITLE, VALUE, NATURE, QUALITY OR
         CONDITION OF THE CONTRIBUTED ASSETS, (2) THE INCOME TO BE DERIVED FROM
         THE CONTRIBUTED ASSETS, (3) THE SUITABILITY OF THE CONTRIBUTED ASSETS
         FOR ANY AND ALL ACTIVITIES AND USES WHICH THE MLP MAY CONDUCT THEREON,
         (4) THE COMPLIANCE OF OR BY THE CONTRIBUTED ASSETS, OR THEIR OPERATIONS
         WITH ANY LAWS (INCLUDING WITHOUT LIMITATION ANY ZONING, ENVIRONMENTAL
         PROTECTION, POLLUTION OR LAND USE LAWS, RULES, REGULATIONS, ORDERS OR
         REQUIREMENTS), OR (5) THE HABITABILITY, MERCHANTABILITY, MARKETABILITY,
         PROFITABILITY OR FITNESS FOR A PARTICULAR PURPOSE OF THE CONTRIBUTED
         ASSETS.

(ii)     THE MLP AND THE OPERATING PARTNERSHIP ACKNOWLEDGE AND AGREE THAT THEY
         HAVE HAD THE OPPORTUNITY TO INSPECT THE CONTRIBUTED ASSETS, AND THAT
         THEY ARE RELYING SOLELY ON THEIR OWN INVESTIGATION OF THE CONTRIBUTED
         ASSETS, AND NOT ON ANY INFORMATION PROVIDED OR TO BE PROVIDED BY POCC
         AND THE SUBSIDIARIES, AND POCC AND THE SUBSIDIARIES ARE NOT LIABLE OR
         BOUND IN ANY MANNER BY ANY VERBAL OR WRITTEN STATEMENTS,
         REPRESENTATIONS OR INFORMATION PERTAINING TO THE CONTRIBUTED ASSETS,
         FURNISHED BY ANY AGENT, EMPLOYEE, SERVANT OR THIRD PARTY.

(iii)    THE MLP AND THE OPERATING PARTNERSHIP ACKNOWLEDGE THAT TO THE MAXIMUM
         EXTENT PERMITTED BY LAW, THE CONTRIBUTION OF THE CONTRIBUTED ASSETS, AS
         PROVIDED FOR HEREIN IS MADE ON AN "AS IS", "WHERE IS" BASIS WITH ALL
         FAULTS AND THE CONTRIBUTED ASSETS, ARE CONTRIBUTED OR DISTRIBUTED AND
         CONVEYED BY POCC AND THE SUBSIDIARIES SUBJECT TO THE FOREGOING. THIS
         PARAGRAPH SHALL SURVIVE SUCH CONTRIBUTION OR DISTRIBUTION AND
         CONVEYANCE OR THE TERMINATION OF THIS AGREEMENT.

(iv)     THE PROVISIONS OF THIS SECTION 4.1 HAVE BEEN NEGOTIATED BY POCC, THE
         SUBSIDIARIES, THE MLP AND THE OPERATING PARTNERSHIP AFTER DUE
         CONSIDERATION AND ARE INTENDED TO BE A COMPLETE EXCLUSION AND NEGATION
         OF ANY REPRESENTATIONS OR WARRANTIES OF POCC AND THE SUBSIDIARIES,
         WHETHER EXPRESS, IMPLIED OR STATUTORY, WITH RESPECT TO THE CONTRIBUTED
         ASSETS, THAT MAY ARISE PURSUANT TO ANY LAW NOW OR HEREAFTER IN EFFECT,
         OR OTHERWISE.

(b)      The contributions of the Contributed Assets, made under this Agreement
         are made with full rights of substitution and subrogation of the
         Operating Partnership, and all persons claiming by, through and under
         the Operating Partnership, to the extent assignable, in and to all
         covenants and warranties by the predecessors-in-title of POCC and the
         Subsidiaries, and with full subrogation of all rights accruing under
         applicable statutes of limitation and all rights of action of warranty

                                       3
<PAGE>

         against all former owners of the Contributed Assets.

(c)      POCC, the Subsidiaries, the MLP, the GP, the Operating Partnership and
         Operating GP agree that the disclaimers contained in this Section 4.1
         are "conspicuous" disclaimers. Any covenants implied by statute or law
         by the use of the words "grant," "convey," "bargain," "sell," "assign,"
         "transfer," "deliver," or "set over" or any of them or any other words
         used in this Agreement or any exhibits hereto are hereby expressly
         disclaimed, waived or negated.

                                   ARTICLE V

                               FURTHER ASSURANCES

         SECTION 5.1 FURTHER ASSURANCES. From time to time after the date
hereof, and without any further consideration, POCC, the Subsidiaries, the GP,
the MLP, the Operating GP and the Operating Partnership shall execute,
acknowledge and deliver all such additional deeds, assignments, conveyances,
instruments, notices, releases, acquittances and other documents, and will do
all such other acts and things, all in accordance with applicable law, as may be
necessary or appropriate more fully and effectively to vest in the Operating
Partnership and the MLP and their successors and assigns beneficial and record
title to the Contributed Assets hereby contributed and assigned to the Operating
Partnership or intended so to be and to more fully and effectively carry out the
purposes and intent of this Agreement.

         SECTION 5.2 OTHER ASSURANCES. From time to time after the date hereof,
and without any further consideration, each of the parties to this Agreement
shall execute, acknowledge and deliver all such additional instruments, notices
and other documents, and will do all such other acts and things, all in
accordance with applicable law, as may be necessary or appropriate to more fully
and effectively carry out the purposes and intent of this Agreement.

                                   ARTICLE VI

                                  MISCELLANEOUS

         SECTION 6.1 HEADINGS; REFERENCES; INTERPRETATION. All article and
section headings in this Agreement are for convenience only and shall not be
deemed to control or affect the meaning or construction of any of the provisions
hereof. The words "hereof," "herein" and "hereunder" and words of similar
import, when used in this Agreement, shall refer to this Agreement as a whole,
including without limitation, all exhibits attached hereto, and not to any
particular provision of this Agreement. All references herein to articles,
sections, and exhibits shall, unless the context requires a different
construction, be deemed to be references to the articles, sections and exhibits
of this Agreement, respectively, and all such Exhibits attached hereto are
hereby incorporated herein and made a part hereof for all purposes. All personal
pronouns used in this Agreement, whether used in the masculine, feminine or
neuter gender, shall include all other genders, and the singular shall include
the plural and vice versa. The use herein of the word "including" following any
general statement, term or matter shall not be construed to limit such
statement, term or matter to the specific items or matters set forth immediately
following such word or to similar items or matters, whether or not non-limiting
language (such as "without limitation," "but not limited to," or words of
similar import) is used with reference thereto, but rather shall be deemed to
refer to all other items or matters that could reasonably fall within the
broadest possible scope of such general statement, term or matter.

         SECTION 6.2 SUCCESSORS AND ASSIGNS. This Agreement shall be binding
upon and inure to the benefit of the parties signatory hereto and their
respective successors and assigns.

         SECTION 6.3 NO THIRD PARTY RIGHTS. The provisions of this Agreement are
intended to bind the parties signatory hereto as to each other and are not
intended to and do not create rights in any other person or confer upon any
other person any benefits, rights or remedies and no person is or is intended to
be a third party beneficiary of any of the provisions of this Agreement.

                                       4
<PAGE>

         SECTION 6.4 COUNTERPARTS. This Agreement may be executed in any number
of counterparts, all of which together shall constitute one agreement binding on
the parties hereto.

         SECTION 6.5 GOVERNING LAW. This Agreement shall be governed by, and
construed in accordance with, the laws of the State of Texas applicable to
contracts made and to be performed wholly within such state without giving
effect to conflict of law principles thereof, except to the extent that it is
mandatory that the law of some other jurisdiction, shall apply.

         SECTION 6.6 SEVERABILITY. If any of the provisions of this Agreement
are held by any court of competent jurisdiction to contravene, or to be invalid
under, the laws of any political body having jurisdiction over the subject
matter hereof, such contravention or invalidity shall not invalidate the entire
Agreement. Instead, this Agreement shall be construed as if it did not contain
the particular provision or provisions held to be invalid, and an equitable
adjustment shall be made and necessary provision added so as to give effect to
the intention of the parties as expressed in this Agreement at the time of
execution of this Agreement.

         SECTION 6.7 AMENDMENT OR MODIFICATION. This Agreement may be amended or
modified from time to time only by the written agreement of all the parties
hereto.

         SECTION 6.8 INTEGRATION. This Agreement, together with that certain
Omnibus Agreement dated of even date herewith, to be entered into by and among
POCC, certain of POCC's subsidiaries, the MLP, the GP, the Operating Partnership
and the Operating GP (the "Omnibus Agreement"), supersedes all previous
understandings or agreements between the parties, whether oral or written, with
respect to its subject matter. This document is an integrated agreement which
contains the entire understanding of the parties. No understanding,
representation, promise or agreement, whether oral or written, other than those
contained in the Omnibus Agreement, is intended to be or shall be included in or
form part of this Agreement unless it is contained in a written amendment hereto
executed by the parties hereto after the date of this Agreement.

                         (Signatures on following page)

                                       5
<PAGE>

         IN WITNESS WHEREOF, this Agreement has been duly executed by the
parties hereto as of the date first above written.

                           PENN OCTANE CORPORATION

                           By:________________________________________
                           Name:______________________________________
                           Title:_____________________________________

                           RIO VISTA GP LLC

                           By:________________________________________
                           Name:______________________________________
                           Title:_____________________________________

                           RIO VISTA ENERGY PARTNERS L.P.

                           By: RIO VISTA GP LLC,
                               its General Partner

                               By:____________________________________
                               Name:__________________________________
                               Title:_________________________________

                           RIO VISTA OPERATING GP LLC

                           By:________________________________________
                           Name:______________________________________
                           Title:_____________________________________

                           RIO VISTA OPERATING PARTNERSHIP L.P.

                           By: Rio Vista Operating GP LLC, its General Partner

                               By: Rio Vista Energy Partners L.P.,
                                   its sole member

                                   By: Rio Vista GP LLC,
                                       its General Partner

                                       By:____________________________________
                                       Name:__________________________________
                                       Title:_________________________________

                                           6
<PAGE>

                                    EXHIBIT A

                              LIST OF SUBSIDIARIES

         Penn-Octane de Mexico, S.A. de C.V.

         Termatsal, S.A. de C.V.

<PAGE>

                                    EXHIBIT B

<PAGE>

                              CONVEYANCE AGREEMENT

         Recording Requested by and When Recorded Return to: Fulbright &
Jaworski L.L.P., 300 Convent St., Suite 2200, San Antonio, Texas, Attn:
Christian G. Herff.

                              CONVEYANCE AGREEMENT

         This Conveyance Agreement (this "Conveyance"), effective as of 12:01
A.M. Eastern Time on ________________, 2003 (the "Effective Date"), is from PENN
OCTANE CORPORATION, a Delaware corporation (herein called "Grantor"), and in
favor of RIO VISTA OPERATING PARTNERSHIP L.P., whose mailing address is 77-530
Enfield Lane, Building D, Palm Desert, CA 92211 (herein called "Grantee").

                                    ARTICLE I

                                 GRANTING CLAUSE

         1.1 GRANTING CLAUSES. Grantor hereby contributes, conveys, assigns,
transfers, delivers, and sets over unto Grantee, its successors and assigns, all
right, title, interests and estate of Grantor in and to the following described
property, to-wit:

         ALL OF THE ASSETS SET FORTH ON SCHEDULE A ATTACHED HERETO

         The property described in this Section 1.1 shall be referred to herein
collectively as the "Subject Property".

         TO HAVE AND TO HOLD the Subject Property, subject to the terms and
conditions hereof, unto Grantee, its successors and assigns, forever.

                                   ARTICLE II

                      ENCUMBRANCES AND WARRANTY DISCLAIMERS

         2.1 PERMITTED ENCUMBRANCES. This Conveyance is made and accepted
expressly subject to (a) all liens, charges, encumbrances, contracts,
agreements, instruments, obligations, defects, restrictions, security interests,
options or preferential rights to purchase, adverse claims, reservations,
exceptions, easements, rights-of-way, conditions, leases, other matters
affecting the Subject Property or to which it is subject; and (b) to all matters
that a current on the ground survey or visual inspection would reflect.

         2.2 CONTRIBUTION AGREEMENT. This Conveyance is expressly made subject
to the terms and conditions of that certain Contribution, Conveyance and
Assumption Agreement dated as of _____________, 2003, among Grantor, Grantee and
the other parties thereto (the "Contribution Agreement"). All capitalized terms
used herein shall have the meanings given to such terms in the Contribution
Agreement, unless otherwise defined herein. Nothing contained in this Conveyance
shall in any way affect the provisions set forth in the Contribution Agreement
nor shall this Conveyance expand or contract any rights or remedies under the
Contribution Agreement. This Conveyance is intended only to effect the transfer
of the Subject Property to Grantee as provided for in the Contribution Agreement
and shall be governed entirely in accordance with the terms and conditions of
the Contribution Agreement. In the event of a conflict between the terms of this
Conveyance and the terms of the Contribution Agreement, the terms of the
Contribution Agreement shall prevail.

         2.3 DISCLAIMER OF WARRANTIES; SUBROGATION. Except as expressly provided
herein or in the Contribution Agreement, this Conveyance is made, and is
accepted by Grantee, without warranty of title, express, implied or statutory,
and without recourse, but with full substitution and subrogation of Grantee, and
all persons claiming by, through, and under Grantee, to the extent assignable,
in and to all covenants and warranties by the predecessors in title of Grantor
and with full subrogation of all rights accruing under applicable statutes of
limitation or prescription and all rights of action of warranty against all
former owners of the Subject Property. Except as expressly provided herein or in
the Contribution Agreement, any covenants implied by statute or by the

                                      D-1
<PAGE>

use of the words "convey", "sell", "assign", "transfer", "deliver", or "set
over" or any of them or any other words used in this Conveyance, are hereby
expressly disclaimed, waived and negated.

                                   ARTICLE III

                                  MISCELLANEOUS

         3.1 FURTHER ASSURANCES. Grantor and Grantee agree to take all such
further actions and to execute, acknowledge and deliver all such further
documents that are necessary or useful in carrying out the purposes of this
Conveyance. So long as authorized by applicable law so to do, Grantor agrees to
execute, acknowledge and deliver to Grantee all such other additional
instruments, notices, affidavits, deeds, conveyances, assignments and other
documents and to do all such other and further acts and things as may be
necessary or useful to more fully and effectively grant, assign, convey,
transfer and deliver to Grantee the Subject Property conveyed hereby or intended
so to be conveyed.

         3.2 SUCCESSORS AND ASSIGNS; NO THIRD PARTY BENEFICIARY. This Conveyance
shall be binding upon, and shall inure to the benefit of, Grantor and Grantee
and their successors and assigns. The provisions of this Conveyance are not
intended to and do not create rights in any other person or entity or confer
upon any other person or entity any benefits, rights or remedies and no person
or entity is or is intended to be a third party beneficiary of any of the
provisions of this Conveyance.

         3.3 GOVERNING LAW. This Conveyance and the legal relations between the
parties shall be governed by, and construed in accordance with, the laws of the
State of Texas, excluding any conflict of law rule which would refer any issue
to the laws of another jurisdiction, except when it is mandatory that the law of
the jurisdiction wherein the Subject Property is located shall apply.

         3.4 HEADINGS; REFERENCES; DEFINED TERMS. All Section headings in this
Conveyance are for convenience only and shall not be deemed to control or affect
the meaning or construction of any of the provisions hereof. The words "hereof",
"herein" and "hereunder" and words of similar import, when used in this
Conveyance, shall refer to this Conveyance as a whole, including, without
limitation, all Schedules and Exhibits attached hereto, and not to any
particular provision of this Conveyance.

         3.5 COUNTERPARTS. This Conveyance may be executed in any number of
counterparts, all of which together shall constitute one agreement binding on
the parties hereto.

         3.6 SEVERABILITY. If any of the provisions of this Conveyance are held
by any court of competent jurisdiction to contravene, or to be invalid under,
the laws of any political body having jurisdiction over the subject matter
hereof, such contravention or invalidity shall not invalidate the entire
agreement. Instead, this Conveyance shall be construed as if it did not contain
the particular provision or provisions held to be invalid and an equitable
adjustment shall be made and necessary provision added so as to give effect to
the intention of the parties as expressed in this Conveyance at the time of
execution of this Conveyance.

                                      D-2
<PAGE>

         IN WITNESS WHEREOF, this Conveyance has been duly executed by the
parties hereto on the dates of the acknowledgments set forth below, to be
effective, however, as of the Effective Date.

                           GRANTOR:

                           PENN OCTANE CORPORATION

                           By:____________________________________
                           Name:__________________________________
                           Title:_________________________________

                           GRANTEE:

                           RIO VISTA OPERATING PARTNERSHIP L.P.

                           By: Rio Vista Operating GP LLC, its General Partner

                               By: Rio Vista Energy Partners L.P.,
                                   its sole member

                                   By: Rio Vista GP LLC,
                                       its General Partner

                                       By:____________________________________
                                       Name:__________________________________
                                       Title:_________________________________

                                      D-3
<PAGE>

THE STATE OF TEXAS        )
                          )
COUNTY OF _________       )

         This instrument was acknowledged before me on the ____ day of ________,
2003, by ______________, ______________ of ______________________, on behalf of
and in [HIS/HER] capacity as __________ of ____________________.

                                  ____________________________________
                                  NOTARY PUBLIC

My Commission Expires: ________________

                                      D-4
<PAGE>

                                   SCHEDULE A

                                      D-5