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                                                                     EXHIBIT 4.1

                             SUPPLEMENTAL INDENTURE

         THIS SUPPLEMENTAL INDENTURE (this "Supplemental Indenture") is dated as
of October 27, 2005 between CITGO PETROLEUM CORPORATION, a Delaware corporation
(the "Company") and J.P. MORGAN TRUST COMPANY, NATIONAL ASSOCIATION (successor
in interest to Bank One, N.A., successor in interest to First National Bank of
Chicago), as trustee ("Trustee"). Capitalized terms used but not defined herein
shall have the meaning given to such terms in the Indenture (as defined below).

         WHEREAS, the Company and the Trustee entered into an Indenture, dated
as of May 1, 1996 (the "Indenture"), providing for the issuance of the Company's
7 7/8% Senior Notes due 2006 (the "Securities");

         WHEREAS, the Company has proposed certain amendments to the Indenture
(the "Proposed Amendments") which, pursuant to Section 902 of the Indenture,
must be approved with the consent of Holders of no less than a majority in
principal amount of all Outstanding Securities;

         WHEREAS, pursuant to the Offer to Purchase and Consent Solicitation
Statement, dated October 13, 2005, and related Consent and Letter of Transmittal
(together, the "Consent Solicitation Statement"), the Company has offered to
purchase the Notes (the "Offer") and solicited the consents of the Holders of
the Notes to the Proposed Amendments, upon the terms and subject to the
conditions set forth therein;

         WHEREAS, the Company has received the valid consents of Holders of more
than a majority in principal amount of the Outstanding Securities in respect of
the Proposed Amendments set forth in this Supplemental Indenture; and

         WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding, and legal agreement, in accordance with
its terms, have been performed and fulfilled and the execution and delivery
hereof have been in all respects duly authorized.

         NOW THEREFORE, in consideration of the premises herein contained and
for other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, it is mutually covenanted and agreed, as follows:

                                    ARTICLE I

                             AMENDMENTS TO INDENTURE

         SECTION 1.01. Amendments to Article Ten. On the date (the "Payment
Date") that the Company pays for all Securities validly tendered and not
withdrawn pursuant to the Offer (unless, prior to that time, the Company has
terminated this Supplemental Indenture as provided


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in Section 2.07 hereof), this Supplemental Indenture shall become operative and
the following Sections of the Indenture, and any corresponding provisions in the
Securities, shall be hereby deleted in their entirety and replaced with
"Intentionally Omitted":

         <Table>
         <Caption>

         EXISTING SECTION NUMBER                                 CAPTION
         -----------------------                                 -------
         <S>                                                     <C>
         Section 1005.........................................   Maintenance of Properties
         Section 1006.........................................   Insurance
         Section 1007.........................................   Payment of Taxes and Other Claims
         Section 1008.........................................   Limitation on Restricted Payments
         Section 1009.........................................   Specified Agreements
         Section 1010.........................................   Transactions with Affiliates
         Section 1011.........................................   Limitation on Liens
         Section 1012.........................................   Limitation on Sale and Leaseback Transactions
         Section 1013.........................................   Restricted and Unrestricted Subsidiaries
         Section 1014.........................................   Reports to Holders
         </Table>

         SECTION 1.02. Amendments to Article Five. On the Payment Date (unless,
prior to that time, the Company has terminated this Supplemental Indenture as
provided in Section 2.07 hereof), this Supplemental Indenture shall become
operative and Section 501 shall be hereby amended and restated to read in its
entirety as follows:

         "SECTION 501. Events of Default. An "Event of Default" occurs with
respect to the Securities of a series if:

         (i)   the Company defaults in any payment of interest on any Security
of such series when it becomes due and payable, and such default continues for a
period of 30 days;

         (ii)  the Company defaults in the payment of the principal of or
premium, if any, on any Security of such series when the same becomes due and
payable at maturity, upon acceleration, required repurchase, or otherwise;

         (iii) the Company fails to comply with any of its agreements or
covenants in, or provisions of, the Securities or this Indenture (other than
clause (i) or (ii) above) and the default continues for the period and after the
notice specified below;

         (iv)  Intentionally Omitted;

         (v)   Intentionally Omitted;

         (vi)  Intentionally Omitted;

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         (vii)  Intentionally Omitted;

         (viii) any other event specified as an "Event of Default" for such
series pursuant to Section 301(xv) hereof occurs.

         A default under clause (iii) of this Section 501 is not an Event of
Default with respect to the Securities of a series until the Trustee notifies
the Company in writing, or the Holders of at least 25% in principal amount of
the Securities of such series then Outstanding notify the Company and after
receipt of the notice. The notice must specify the default, demand that it be
remedied and state that the notice is a "Notice of Default". Such notice to the
Company shall be given by the Trustee if so requested in writing by the Holders
of at least 25% of the principal amount of the Securities of such series then
Outstanding."

         SECTION 1.03. Amendments to Section 801. On the Payment Date (unless,
prior to that time, the Company has terminated this Supplemental Indenture as
provided in Section 2.07 hereof), this Supplemental Indenture shall become
operative and Section 801 shall be hereby amended and restated to read in its
entirety as follows:

         "SECTION 801. When Companies May Merge, etc. The Company shall not, and
shall not permit any Restricted Subsidiary to, merge or consolidate with or into
any other Person (other than a merger of a Restricted Subsidiary into the
Company, or a merger of a Restricted Subsidiary with another Restricted
Subsidiary) or sell, transfer, assign, lease, convey or otherwise dispose of all
or substantially all of its Property or assets to any Person, unless (i) the
entity formed by or surviving any such consolidation or merger (if the Company
is a party to the transaction and is not the surviving entity) or to which such
sale, transfer or conveyance is made (the "Surviving Entity") shall be a solvent
corporation, limited partnership or limited liability company organized and
existing under the laws of the United States or a State thereof or the District
of Columbia and such Surviving Entity expressly assumes, by supplemental
indenture satisfactory to the Trustee, executed and delivered to the Trustee in
accordance with the terms of this Indenture by such Surviving Entity, the due
and punctual payment of the principal of, premium, if any, and interest on all
the Securities, according to their tenor, and the due and punctual performance
and observance of all of the covenants and conditions of this Indenture to be
performed by the Company; and (ii) immediately before and after giving effect to
such transaction (and treating any Indebtedness which becomes an obligation of
the Surviving Entity or any Restricted Subsidiary as a result of such
transaction as having been Incurred by the Surviving Entity or such Restricted
Subsidiary at the time of such transaction), no Default or Event of Default
shall have occurred and be continuing."

                                   ARTICLE II

                                  MISCELLANEOUS

         SECTION 2.01. Instruments To Be Read Together. This Supplemental
Indenture is an indenture supplemental to and in implementation of the
Indenture, and said Indenture and this Supplemental Indenture shall henceforth
be read together.

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         SECTION 2.02. Confirmation. The Indenture, as amended and supplemented
by this Supplemental Indenture, is in all respects confirmed and preserved.

         SECTION 2.03. Terms Defined. Capitalized terms used in this
Supplemental Indenture have been inserted for convenience of reference only, and
are not to be considered a part hereof and shall in no way modify or restrict
any of the terms and provisions hereof.

         SECTION 2.04. Headings. The headings of the Articles and Sections of
this Supplemental Indenture have been inserted for convenience of reference
only, and are not to be considered a part hereof and shall in no way modify or
restrict any of the terms and provisions hereof.

         SECTION 2.05. Governing Law. This Supplemental Indenture shall be
governed by and construed in accordance with the laws of the State of New York.

         SECTION 2.06. Counterparts. This Supplemental Indenture may be executed
in any number of counterparts, each of which so executed shall be deemed to be
an original, but all such counterparts shall together constitute but one and the
same instrument.

         SECTION 2.07. Effectiveness; Termination. The provisions of this
Supplemental Indenture will take effect immediately upon its execution and
delivery by the Trustee in accordance with the provisions of Sections 104, 903
and 904 of the Indenture; provided, that the amendments to the Indenture set
forth in Article I of this Supplemental Indenture shall become operative as
specified in Article I hereof. Prior to the Payment Date, the Company may
terminate this Supplemental Indenture upon written notice to the Trustee. Upon
receipt by the Trustee of any such notice of termination, this Supplement
Indenture shall be deemed terminated and of no further force or effect.

         SECTION 2.08. Acceptance by Trustee. The Trustee accepts the amendments
to the Indenture effected by this Supplemental Indenture and agrees to execute
the trusts created by the Indenture as hereby amended, but only upon the terms
and conditions set forth in the Indenture.

         SECTION 2.09. Responsibility of Trustee. The recitals contained herein
shall be taken as the statements of the Company, and the Trustee assumes no
responsibility for their correctness. The Trustee makes no representations as to
the validity or sufficiency of this Supplemental Indenture.

                            [Signature Pages Follow]

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         IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed, all as of the date first written above.



                                       CITGO PETROLEUM CORPORATION


                                       By:  /s/ Philip J. Reedy
                                            ------------------------------------
                                            Name: Philip J. Reedy
                                            Title: Treasurer


                                       J.P. MORGAN TRUST COMPANY, NATIONAL
                                       ASSOCIATION


                                       By:  /s/ B. Impala
                                            ------------------------------------
                                            Name: B. Impala
                                            Title: Assistant Vice President






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