<PAGE>

                                                                     EXHIBIT 4.2

                             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, 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 October 22, 2004 (the "Indenture"), providing for the issuance of the
Company's 6% Senior Notes due 2011 (the "Notes");

         WHEREAS, the Company has proposed certain amendments to the Indenture
(the "Proposed Amendments") which, pursuant to Section 9.02 of the Indenture,
must be approved with the written consent of Holders of no less than a majority
in principal amount of all outstanding Notes;

         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 Notes 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 4. On the date (the "Payment Date")
that the Company pays for all Notes validly tendered and not withdrawn pursuant
to the Offer (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 the following


<PAGE>
Sections of the Indenture, and any corresponding provisions in the Notes, shall
be hereby deleted in their entirety and replaced with "Intentionally Omitted":

<Table>
<Caption>
         EXISTING SECTION NUMBER                                 CAPTION
         -----------------------                                 -------
         <S>                                                     <C>
         Section 4.02.........................................   SEC Reports
         Section 4.03.........................................   Limitation on Indebtedness
         Section 4.04.........................................   Limitation on Restricted Payments
         Section 4.05.........................................   Limitation on Restrictions on Distributions from
                                                                 Restricted Subsidiaries
         Section 4.06.........................................   Limitation on Sales of Assets and Subsidiary Stock
         Section 4.07.........................................   Limitation on Affiliate Transactions
         Section 4.08.........................................   Limitation on Issuance of Guarantees of Indebtedness
         Section 4.09.........................................   Change of Control Triggering Event
         Section 4.10.........................................   Limitation on Liens
         Section 4.13.........................................   Investment Grade Covenants
</Table>

         SECTION 1.02. Amendments to Article 5. 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 Sections 5.01 and 5.02 shall be hereby amended and restated to
read in their entirety as follows:

         "Section 5.01. When Company May Merge or Transfer Assets. The Company
shall not consolidate with or merge with or into, or convey, transfer or lease,
in one transaction or a series of transactions, directly or indirectly, all or
substantially all of its assets to, any Person, unless:

         (1) the resulting, surviving or transferee Person (the "Successor
Company") shall be a Person organized and existing under the laws of the United
States of America, any State thereof or the District of Columbia and the
Successor Company (if not the Company) shall expressly assume, by an indenture
supplemental hereto, executed and delivered to the Trustee, in form satisfactory
to the Trustee and executed in accordance with the terms of this Indenture, all
the obligations of the Company under the Notes and this Indenture; and

         (2) immediately after giving effect to such transaction (and treating
any Indebtedness which becomes an obligation of the Successor Company or any
Subsidiary as a result of such transaction as having been Incurred by such
Successor Company or such Subsidiary at the time of such transaction), no
Default shall have occurred and be continuing.

                                      -2-
<PAGE>

         (3)  Intentionally Omitted.

         For purposes of this covenant, the sale, lease, conveyance, assignment,
transfer or other disposition of all or substantially all of the properties and
assets of one or more Subsidiaries of the Company, which properties and assets,
if held by the Company instead of such Subsidiaries, would constitute all or
substantially all of the properties and assets of the Company on a consolidated
basis, shall be deemed to be the transfer of all or substantially all of the
properties and assets of the Company.

         The Successor Company shall be the successor to the Company and shall
succeed to, and be substituted for, and may exercise every right and power of,
the Company under this Indenture, and the predecessor Company, except in the
case of a lease, shall be released from the obligation to pay the principal of
and interest on the Notes.

         Section 5.02. When Subsidiary Guarantor May Merge or Transfer Assets.
The Company shall not permit any Subsidiary Guarantor to consolidate with or
merge with or into, or convey, transfer or lease, in one transaction or series
of transactions, all or substantially all of its assets to any Person unless:

         (1) the resulting, surviving or transferee Person (if not such
Subsidiary) shall be a Person organized and existing under the laws of the
jurisdiction under which such Subsidiary was organized or under the laws of the
United States of America, or any State hereof or the District of Columbia, and
such Person shall expressly assume, by an amendment or supplemental indenture to
this Indenture, in a form acceptable to the Trustee and executed in accordance
with the terms of this Indenture, all the obligations of such Subsidiary, if
any, under its Subsidiary Guaranty; provided, however, that the provisions of
this Section 5.02(1) shall not apply in the case of a Subsidiary Guarantor that
has been disposed of in its entirety to another Person (other than to the
Company or an Affiliate of the Company), whether through a merger, consolidation
or sale of Capital Stock or assets; and

         (2) immediately after giving effect to such transaction or transactions
on a pro forma basis (and treating any Indebtedness which becomes an obligation
of the resulting, surviving or transferee Person as a result of such transaction
as having been issued by such Person at the time of such transaction), no
Default shall have occurred and be continuing.

         (3) Intentionally Omitted.


         SECTION 1.03. Amendments to Article 6. 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 6.01 shall be hereby amended and restated to read in its
entirety as follows:

         "Section 6.01. Events of Default. An "Event of Default" occurs if:

         (1) the Company defaults in any payment of interest on any Note when
the same becomes due and payable, and such default continues for a period of 30
days;

                                      -3-
<PAGE>

         (2) the Company defaults in the payment of the principal of any Note
when the same becomes due and payable at its Stated Maturity, upon optional
redemption, upon required purchase, upon declaration of acceleration or
otherwise;

         (3) the Company fails to comply with Section 5.01;

         (4) Intentionally Omitted;

         (5) The Company or any Subsidiary Guarantor fails to comply with any of
its other agreements in this Indenture and such failure continues for 60 days
after the notice specified below;

         (6) Intentionally Omitted;

         (7) Intentionally Omitted;

         (8) Intentionally Omitted;

         (9) Intentionally Omitted;

         (10) Any Subsidiary Guaranty ceases to be in full force and effect
(other than in accordance with the terms of this Indenture and such Subsidiary
Guaranty) or any Subsidiary Guarantor denies or disaffirms its obligations under
its Subsidiary Guaranty.

         The foregoing will constitute Events of Default whatever the reason for
any such Event of Default and whether it is voluntary or involuntary or is
effected by operation of law or pursuant to any judgment, decree or order of any
court or any order, rule or regulation of any administrative or governmental
body.

         The term "Bankruptcy Law" means Title 11, United States Code, or any
similar Federal or state law for the relief of debtors. The term "Custodian"
means any receiver, trustee, assignee, liquidator, custodian or similar official
under any Bankruptcy Law.

         A Default under clause (5) shall not constitute an Event of Default
until the Trustee or the Holders of at least 25% in principal amount of the
outstanding Notes notify the Company in writing of the Default and the Company
does not cure such Default within the time specified after receipt of such
notice. Such notice must specify the Default, demand that it be remedied and
state that such notice is a "Notice of Default."

         The Company shall deliver to the Trustee, within 30 days after the
occurrence thereof, written notice in the form of an Officers' Certificate of
any event which with the giving of notice or the lapse of time would become and
Event of Default under clause (5), its status and what action the Company is
taking or proposes to take with respect thereto."

                                      -4-
<PAGE>


                                   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.

         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 9.02 and
9.04 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.

                                      -5-
<PAGE>
         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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