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**Exhibit      10.11**

|   | **Re:** | **Amendment                of Promissory Note(s) of Penn Octane Corporation Due December 15,                2005 and                Related Agreements** |
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|   | i.) | The                  promissory note(s) originally issued by Penn Octane in connection                  with the                  private placement on or around December 17, 1999 (the “Original Notes”),                  as amended (the “Restructured Notes”),                  and/or |
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|   | ii.) | The                  new promissory note(s) originally issued by Penn Octane contemporaneously                  with the restructuring of the Original Notes, as amended (the “New                  Notes”), and/or |
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|   | iii.) | The                  promissory note originally issued in June 2002 for $200,000 and                  December                  2002 for $300,000, as amended (the “Additional Notes”),                  and/or |
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|   | iv.) | The                  promissory notes originally issued January 16, 2004 to various                  holders                  (the “$280,000 Notes”). |
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|   | (a) | The                Noteholders hereby agree to extend the payment due date for the Interest                Payments and Additional Fees otherwise due June 15, 2005 and                September 15, 2005 until December 15, 2005 or any earlier                maturity date of the Promissory Notes (the “Maturity Date”). The                Noteholders hereby acknowledge and agree that there are no defaults                existing under the Promissory Notes as of the date                hereof. |
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|   | (b) | Any                amounts deferred pursuant to Paragraph (a) above shall accrue interest                at                a rate of 16.5% per year from the date the payments were otherwise                due                until the date that the deferred amounts are                paid. |
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|   | (c) | All                accrued interest and principal shall be due and payable on                December 15, 2005; provided, however, that if Penn Octane and/or Rio                Vista Energy Partners L.P. (“Rio Vista”) completes a sale of any assets                resulting in cash proceeds of more than $2 million to Penn Octane                and/or Rio Vista before December 15, 2005, then the cash proceeds                in                excess of $2 million (but only such excess) shall be first applied to                satisfaction of Penn Octane’s obligations under the Promissory Notes                before other uses of such excess                proceeds. |
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|   | (d) | The                Noteholders shall be entitled to receive an additional fee equal                to 5% of                the principal amount outstanding on the date hereof, payable on December                15, 2005 or the Maturity Date, whichever first occurs. |
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|   | (e) | Jerome                  B. Richter (“Richter”) agrees to deliver to the Law Offices of Kevin Finck                  (“Finck”), within thirty (30) days following the date hereof, 250,000                  common units of Rio Vista (the “Pledged Units”) in connection with                  Richter’s previous pledge of 2,000,000 shares of Penn Octane common stock                  (the “Pledged Shares”) to guaranty the obligations evidenced by the                  Promissory Notes. In addition, the Noteholders confirm that, subject                  to                  the Escrow Agreement (defined below), Finck is authorized to act                  as                  collateral agent on behalf of the Noteholders with respect to the                  Pledged                  Shares, currently held by Investec (US) Incorporated (“Investec”), as                  successor to PMG Capital Corp., and the Pledged Units. The Noteholders                  agree to cause Investec to deliver the Pledged Shares to Finck                  within                  thirty (30) days following the date hereof. Penn Octane shall instruct                  Finck to provide to Amir Ecker, as representative of the Noteholders,                  with                  confirmation of receipt of the Pledged Securities within fifteen                  (15) days                  following receipt of the Pledged Securities. The Noteholders hereby                  agree                  and consent to the Escrow Agreement dated September 30, 2005, by and                  among Richter, Penn Octane, the Noteholders, and Finck, in the                  form                  attached as Exhibit                  B                  (the “Escrow Agreement”), and shall execute and deliver the Escrow                  Agreement together with this amendment letter. The Noteholders                  agree to                  execute and deliver any further documents reasonably requested                  by Finck in                  connection with the Escrow                  Agreement. |
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|   | (f) | Penn                Octane agrees that it will not incur any additional severance payment                obligation to Richter until the Promissory Notes have been fully                paid. |
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|   | (g) | The                Noteholders agree that Rio Vista and its subsidiaries shall be allowed                to                pledge and deliver certain assets (the “Assets”) of Rio Vista and its                subsidiaries in connection with a loan of $1.3 million to Rio Vista                and                its subsidiaries as provided in Section 3.1 of the Purchase and Sale                Agreement dated August 15, 2005, between Rio Vista Operating Parternship                L.P. and TransMontaigne Product Services Inc. (“TPSI”). The Noteholders                further agree that any and all security interests of the Noteholders                in                the Assets (i) are hereby subordinated to any and all security interests                of TPSI in the Assets, and (ii) shall be released if the Assets are                delivered to TPSI. |
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|   | (h) | Penn                Octane agrees that it will be responsible for any legal fees reasonably                incurred by the Noteholders in connection with the enforcement of                any                terms of the Promissory Notes. |
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| Very                truly yours,                                                                                                                                                                                  |                                                      |
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|                                                                                                                                                                                                              |                                                      |   |
| Penn                Octane Corporation                                                                                                                                                                            |                                                      |
|                                                                                                                                                                                                              |                                                      |   |
|                                                                                                                                                                                                              |                                                      |   |
| By:                                                                                                                                                                                                               | /s/              Ian T. Bothwell                          |   |
| Its:                                                                                                                                                                                                              | Vice                President and Chief Financial Officer |   |
|                                                                                                                                                                                                              |                                                      |   |
|                                                                                                                                                                                                              |                                                      |   |
| For                purposes of the first sentence of paragraph (e) above, Jerome B.                Richter                has executed this amendment letter as of the date first set forth                above. |
|                                                                                                                                                                                                              |                                                      |   |
|                                                                                                                                                                                                              |                                                      |   |
| /s/              Jerome B. Richter                                                                                                                                                                                |                                                      |
| Jerome                B. Richter                                                                                                                                                                                  |                                                      |

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