**Exhibit 10.52****GUARANTY & AGREEMENT**Guaranty and Agreement (“Guaranty”) between the Guarantor named below and RZB FINANCELLC (together with its successors and assigns, “RZB”), dated as of July 26, 2007.**1.** **Special Terms**   The following terms and provisions shall apply to this Guaranty; themeaning of any term in this or other sections of this Guarantyexpressed in the singular shall apply, mutatismutandis, to the sameterm expressed in the plural and viceversa.   **Borrower**: Rio Vista Energy Partners L.P., a Delaware limitedpartnership   **Guarantor**: Penn Octane Corporation, a Delawarecorporation   **Guaranteed Obligations**: All indebtedness, liabilities and obligationsof 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**: Loan Agreement, dated as of July 26, 2007 (as amended, modified or supplemented fromtime to time, the “Loan Agreement”), General Security Agreement, dated July 26, 2007 asamended, modified or supplemented from time to time, and Promissory Note, dated July 26,2007 as amended, modified or supplemented from time to time. **2.** **Guaranty** 2.1 Continuing Guaranty of Payment. In consideration of RZB agreeing to the LoanDocuments and/or extending or continuing credit to the Borrower in connection therewith, theGuarantor irrevocably, absolutely and unconditionally guarantees to RZB the payment when due ofall 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 theBorrower or the Guarantor,  2.c. the giving of notice by RZB to the Borrower or Guarantor of the occurrence of anydefault by the Guarantor hereunder (including any material misrepresentation by theGuarantor 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 Documentsor the Guarantor shall disaffirm any of its obligations hereunder, or  2.f.  the Borrower or the Guarantor shall admit in writing its inability to pay its debts asthey become due.This is a guaranty of payment rather than of collection; this is also a continuing guaranty andall liabilities to which this Guaranty applies, or may apply, under the terms hereof shall bepresumed to have been created in reliance hereon.2.2 Nature of Obligations. The obligations of the Guarantor to make payments to RZBhereunder are direct and primary obligations which shall not be discharged for any reason untilRZB has been indefeasibly paid in full. Without limiting the generality of the foregoing, theobligations 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 LoanDocuments or Guaranteed Obligations or any defense which the Borrower may have with respectthereto,  2.b.  the existence or absence of any legal action to enforce the Guaranteed Obligations orthe Loan Documents or any security therefor, the issuance of any judgment therefor or theexecution of any such judgment, or  2.c.  any other circumstance which might otherwise constitute a defense available to ordischarge of, a guarantor or surety of any type.This guaranty is several and independent of, and may be enforced regardless of, any otherobligation (direct or contingent) of the Guarantor or any other “Person” (such term to includeany person or legal or governmental entity, association, agency or instrumentality) with respectto the Guaranteed Obligations.2.3 Payments. All payments by the Guarantor hereunder shall be made to RZB withoutset-off, recoupment, deduction or counterclaim at its office set forth below (or as RZB mayotherwise direct) in lawful currency of the United States of America and in immediatelyavailable funds. Without limiting the foregoing, all payments hereunder shall be made free andclear 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 behalfof any government or any political subdivision or agency thereof or therein. If any such taxes,duties or charges are so levied or imposed on any such payment, the Guarantor will makeadditional 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 theamount provided for herein. The Guarantor shall furnish promptly to RZB official receiptsevidencing the payment of any such taxes, duties or charges paid by the Guarantor.
 
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**3.** **Special Agreements of Guarantor**3.1 Subordination. Subject to the next following sentence of this Section 3.1: 3.a. all claims of the Guarantor against the Borrower shall be subject and subordinate tothe prior payment to RZB of all Guaranteed Obligations and all obligations of the Guarantorhereunder, and  3.b.  the Guarantor shall not be entitled to receive any payment or exercise any set-off inrespect of any such claim and, to the extent any such payment is received (whetherdirectly, by way of dividend in bankruptcy, set-off or otherwise), the Guarantor willforthwith 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 GuaranteedObligations and, until so delivered, the same shall be held in trust as the property ofRZB.Notwithstanding the foregoing, except as provided in Section 3.2, until the occurrence of anydefault or event of default under the Loan Documents or this Guaranty, or any demand for paymentof any of the Guaranteed Obligations, the Guarantor may receive and retain payment in respect ofany obligation owed to it by the Borrower. If the Guarantor fails to make any necessaryendorsement or assignment of any instrument of payment to which RZB is entitled, RZB and any ofits officers or employees are hereby irrevocably authorized to make the same on behalf of theGuarantor.3.2 No Subrogation. Until indefeasible payment in full of the Guaranteed Obligationsand termination of the Loan Agreement, the Guarantor hereby waives any right of subrogation thatit may have with respect to any payment that it may have made to RZB hereunder.3.3 No Contribution. Until indefeasible payment in full of the Guaranteed Obligationsand termination of the Loan Agreement, the Guarantor agrees that it shall have no legal orequitable right or claim (by way of indemnification, contribution or otherwise) against anysubsidiary or affiliate of Guarantor which has issued a guaranty to RZB in respect of theGuaranteed Obligations.3.4 Waivers. Except to the extent required by law which cannot be waived, the Guarantorwaives 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 nonpaymentof any such liabilities, suit or taking other action or making any demand by RZB against, andany other notice to, any party liable thereon (including the Guarantor). The Guarantor agreesthat RZB may at any time and from time to time, upon or without any terms or conditions and inwhole 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 liabilityincurred directly or indirectly in respect thereof, and this Guaranty shall apply to theGuaranteed Obligations so changed, extended, renewed or altered,  3.b.  fail to record, perfect or protect, or sell, exchange, release, surrender, realize uponor otherwise deal with in any manner and in any order, any property or Person whatsoever atany time securing or guaranteeing the Guaranteed Obligations or any liabilities (includingany 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 thereforor any liability (including any of those hereunder) incurred directly or indirectly inrespect thereof or hereof, and may subordinate the payment of all or any part of theGuaranteed Obligations to the payment of any other liability (whether due or not) of theBorrower to the creditors of the Borrower (including RZB),  3.e.  apply any sums by whomsoever paid or howsoever realized to any liability or liabilitiesof the Borrower to RZB regardless of what liability or liabilities of the Borrower remainunpaid, provided that payments by the Guarantor pursuant to this Guaranty shall be appliedto 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 amendany provision of, the Loan Documents, and/or  3.g. increase the amount of indebtedness of the Borrower to RZB, whether under the LoanDocuments or otherwise.It is understood and agreed that RZB may take any such action without the consent of, or noticeto, the Guarantor, without incurring responsibility to the Guarantor, and without impairing orreleasing the obligations of the Guarantor hereunder.3.5 Amounts Reclaimed. If claim is made upon RZB for repayment or recovery of anyamount received on account of any of the Guaranteed Obligations and RZB repays all or part ofsaid amount by reason of: 3.a.  any judgment, decree or order of any court, administrative body or trustee inbankruptcy (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 suchjudgment, decree, order, settlement or compromise or otherwise) shall be binding upon theGuarantor, notwithstanding any cancellation of the Loan Documents or this Guaranty, and theGuarantor shall be and remain liable to RZB hereunder for the amount so repaid or recovered tothe same extent as if such amount had never been received by RZB.
 
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3.6 Interest. If the Guarantor fails to pay when due any obligation hereunder, then, tothe extent permitted by law, such obligation shall bear interest, payable on demand, from thedue date thereof until paid at a fluctuating rate per annum equal to 2% in excess of the BaseLending Rate (ashereinafter defined); provided that no such additional interest shall be payable in respect ofany 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 announcedby 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 dayand (ii) the Federal Funds Rate (as hereinafter defined) in effect on such day plus 1/2%. (SuchBase Lending Rate is not necessarily intended to be the lowest rate of interest charged by theBank in connection with extensions of credit.) Each change in the Base Lending Rate shall resultin a corresponding change in the interest rate and such change shall be effective on theeffective date of such change in the Base Lending Rate.The term “Federal Funds Rate” means, for any day, the overnight federal funds rate inNew York City, as published for such day (or, if such day is not a New York Business Day, forthe 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 NewYork Business Day, the average of the quotations for such day on overnight federal fundstransactions in New York City received by RZB or the Bank from three federal funds brokers ofrecognized standing selected by RZB or the Bank.3.7 INTENTIONALLY OMITTED3.8 Information. The Guarantor will promptly furnish to RZB such information regardingits and the Borrower’s business, affairs and financial condition as RZB may from time to timereasonably request.3.9 Secured Obligations. The Guarantor shall not grant, create, assume, incur or sufferto exist a security interest in or lien or encumbrance upon any of its property for the purposeof securing the obligations of the Borrower and/or itself to any other Person (other than to theLender or as otherwise expressly permitted under the Loan Agreement) unless such Person shallhave 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 orcontinue the credit provided thereby and to accept this Guaranty, theGuarantor makes the following representations, warranties and agreementswhich 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 thejurisdiction 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 enterinto 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 otheraction required by its organizational documents, and  4.b.  do not contravene any existing law or any legal order applicable to, or license orpermit granted to, the Guarantor, or any agreement or instrument to which the Guarantor isa party or to which it or any of its assets is subject or any provision of the Guarantor’sorganizational documents.This Guaranty is the legal, valid and binding obligation of the Guarantor, enforceable inaccordance with its terms.4.3 Governmental Approvals. Neither any action by or with any governmental or publicbody 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 theentering into, performance or enforcement of this Guaranty (collectively, “GovernmentalApprovals”), except such as has been obtained or taken and with respect to which a copy or othersatisfactory evidence thereof has been furnished to RZB. The Guarantor will maintain allrequisite Governmental Approvals until the Loan Documents are terminated and the GuaranteedObligations and all of its obligations hereunder are paid in full.4.4 Financial Condition. The most recent year-end financial statements of the Guarantorfurnished to RZB prior to the date hereof have been prepared in accordance with generallyaccepted accounting principles consistently applied and fairly present the financial conditionand the results of operations of the Guarantor as at the end of and for the reporting periodcovered thereby. There are no material liabilities or any material unrealized or anticipatedlosses from unfavorable commitments which are not disclosed in such financial statements. Therehas been no material adverse change in the operations, business or financial condition of theGuarantor from that set forth in such financial statements; and there are no legal proceedingspending or, to the knowledge of the Guarantor threatened, against or affecting the Guarantorwhich 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 theInvestment Company Act of 1940, as amended (the “Act”), and the entering into of this Guarantyand 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 acertificate of an authorized officer of the Guarantor wherein the Guarantor shall reaffirm toRZB the continuing truth and validity of the representations and warranties set forth in theforegoing Section 4. The failure of the Guarantor to deliver and/or RZB to demand such deliveryof the foregoing certificate shall in no way affect or invalidate the continuing nature of therepresentations and warranties set forth in the foregoing Section 4.**5.** **Miscellaneous**5.1 Payment of Expenses. The Guarantor agrees to pay all out-of-pocket costs andexpenses of RZB arising in connection with its administration and enforcement of, orpreservation of its rights under, this Guaranty (including, without limitation, the reasonablefees and expenses of counsel for RZB), and all stamp taxes (including interest and penalties, ifany) which may be payable in respect of this Guaranty or of any modification of this Guaranty.
 
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5.2 Modification. This Guaranty may be modified or waived only by an instrument inwriting 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 PRINCIPLESOF CONFLICTS OF LAW).5.4 Notices. Communications given to any party in connection with this Guaranty shallbe in English and in writing and shall be effective when delivered at its address set forthherein, as the same may be changed by written notice to the other party. Written communicationsmay be in any form of writing howsoever transmitted.5.5 Waiver. RZB’s rights, powers, privileges and remedies under this Guaranty orapplicable law are cumulative and not exclusive and shall not be waived, precluded or limited byany failure or delay in the exercise thereof or by RZB’s exercise, or partial exercise, of anythereof or by any course of dealing between the Guarantor and RZB. No notice to or demand onthe Guarantor in any case shall entitle the Guarantor to any other or further notice or demandin similar or other circumstances or constitute a waiver of the right of RZB to any other orfurther action in any circumstance without notice or demand.5.6 Descriptive Headings. The descriptive headings used in this Guaranty are forconvenience only and shall not be deemed to affect the meaning or construction of any provisionhereof.5.7 Benefit of Guaranty. This Guaranty shall be binding upon the Guarantor and itssuccessors and assigns and shall inure to the benefit of, and be enforceable by, RZB and itssuccessors and assigns and, in particular, any holder or assignee from time to time of the LoanDocuments; provided that the Guarantor may not assign any of its rights or obligations hereunderwithout the prior written consent of RZB.5.8 Set-Off. RZB is authorized at any time and from time to time, without notice to theGuarantor or to any other Person, any such notice being hereby expressly waived, to set off andapply any and all deposits (general or special) and any other indebtedness at any time held orowing by RZB to or for the credit or the account of the Guarantor against and on account of theobligations of the Guarantor under this Guaranty, irrespective of whether or not RZB shall havemade any demand hereunder or any demand for payment of any Guaranteed Obligation and althoughsaid obligations, liabilities or claims, or any of them, shall be contingent or unmatured, andthe 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 notentitled 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 wherevercommenced to enforce or collect upon this Guaranty.5.10 Survival. The provisions of Sections 3.5, 5.1 and 5.11 shall survive thetermination and cancellation of this Guaranty and, after cancellation and return to theGuarantor, 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, VOLUNTARYAND INTENTIONALLY WAIVES ANY AND ALL RIGHTS IT MAY HAVE TO A TRIAL BY JURY IN RESPECT OF ANYLITIGATION BASED ON, OR ARISING OUT OF, UNDER, OR IN CONNECTION WITH THIS GUARANTY OR ANY OTHERDOCUMENT OR AGREEMENT EXECUTED IN CONNECTION HEREWITH, OR ANY COURSE OF CONDUCT, COURSE OFDEALING, STATEMENTS (WHETHER VERBAL OR WRITTEN), OR ACTIONS OF THE GUARANTOR, THE BORROWER ORRZB. THIS PROVISION IS A MATERIAL INDUCEMENT FOR RZB’S EXTENDING TO THE BORROWER THE CREDITFACILITY TO WHICH THIS GUARANTY RELATES.5.12 Consent to Jurisdiction. The Guarantor hereby agrees that ANY LEGAL ACTION ORPROCEEDING AGAINST THE GUARANTOR WITH RESPECT TO THIS GUARANTY OR ANY LOAN DOCUMENT MAY BEBROUGHT IN THE COURTS OF THE STATE OF NEW YORK IN THE CITY OF NEW YORK OR OF THE UNITED STATESOF AMERICA FOR THE SOUTHERN DISTRICT OF NEW YORK, as RZB may elect, and, by execution anddelivery hereof, the Guarantor accepts and consents to, for itself and in respect of itsproperty, generally and unconditionally, the jurisdiction of the aforesaid courts and agreesthat such jurisdiction shall be exclusive, unless waived by RZB in writing, with respect to anyaction or proceeding brought by it against RZB and any question relating to usury. Nothingherein shall limit the right of RZB to bring proceedings against the Guarantor in the courts ofany other jurisdiction. Service of process out of any such courts may be made by mailing copiesthereof by registered or certified mail, postage prepaid, to the Guarantor at its address fornotices as specified herein and will become effective 30 days after such mailing. The Guarantoragrees that Sections 5-1401 and 5-1402 of the General Obligations Law of the State of New Yorkshall 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 saidcourt on the basis of, forumnonconveniens.
 
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IN WITNESS WHEREOF, the Guarantor has caused this Guaranty to be executed and delivered by its dulyauthorized officer(s) as of the date first above written.      **Name of Guarantor**:        PENN OCTANE CORPORATION         By: