AMENDMENT NO. 1 TO RECEIVABLES PURCHASE AGREEMENT

Exhibit 4.23(a)

AMENDMENT NO. 1 TO RECEIVABLES PURCHASE AGREEMENT

AMENDMENT dated as of June 25, 2004 to the Receivables Purchase Agreement dated as of December 17, 2003 (the “Receivables Agreement”) among EQUISTAR RECEIVABLES II, LLC, a Delaware limited liability company (the “Seller”), EQUISTAR CHEMICALS, LP, a Delaware limited partnership (the “Servicer”), the PURCHASERS from time to time party thereto, CITICORP USA, INC., as co-asset agent and administrative agent for the Purchasers (the “Agent”), BANK ONE, NA, CREDIT SUISSE FIRST BOSTON, and JPMORGAN CHASE BANK, as co-documentations agents, and BANK OF AMERICA, N.A., as co-asset agent.

The parties hereto agree as follows:

SECTION 1. Defined Terms; References. Unless otherwise specifically defined herein, each term used herein which is defined in the Receivables Agreement has the meaning assigned to such term in the Receivables Agreement. Each reference to “hereof”, “hereunder”, “herein” and “hereby” and each other similar reference and each reference to “this Agreement” and each other similar reference contained in the Receivables Agreement shall, after this Amendment becomes effective, refer to the Receivables Agreement as amended hereby.

SECTION 2. Amended Definition. The definition of “Eligible Receivable” is hereby amended by (i) deleting the words “Millennium or” in the first paragraph thereof, (ii) deleting the words “their respective” in the first paragraph thereof and inserting the word “its” in the place thereof, (iii) deleting the words “Millennium or” in paragraph (c) thereof, and (iv) deleting the words “their respective” in paragraph (c) thereof and inserting the word “its” in the place thereof.

SECTION 3. Representations of Seller and Servicer. Each of the Seller and the Servicer represent and warrant that (i) their respective representations and warranties set forth in Article IV of the Receivables Agreement will be true in all material respects on and as of the Amendment Effective Date (as defined below) with the same effect as though made on and as of such date, except to the extent such representations and warranties expressly relate to an earlier date and (ii) no Potential Event of Termination or Event of Termination will have occurred and be continuing on such date.

SECTION 4. Governing Law. This Amendment shall be governed by and construed in accordance with the laws of the State of New York.

SECTION 5. Counterparts. This Amendment may be signed in any number of counterparts, each of which shall be an original, with the same effect as if the signatures thereto and hereto were upon the same instrument.

SECTION 6. Effectiveness. This Amendment shall become effective on the first date when, and simultaneously with the time upon which, the following conditions are met (the “Amendment Effective Date”):

(a) the Agent shall have signed a counterpart hereof and shall have received counterparts hereof signed by each of the Purchasers, the Seller and the Servicer (or, in the case of any party as to which an executed counterpart shall not have been received, the Agent shall have received in form satisfactory to it facsimile or other written confirmation from such party of execution of a counterpart hereof by such party); and

(b) Lyondell’s acquisition of Millennium shall have been consummated.

Promptly after the Amendment Effective Date occurs, the Agent shall notify the Seller, the Servicer, the Purchasers and the other Facility Agents thereof, and such notice shall be conclusive and binding on all parties hereto.

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