E XHIBIT 10.1

OMNIBUS ASSUMPTION AND ASSIGNMENT AGREEMENT

THIS OMNIBUS ASSUMPTION AND ASSIGNMENT AGREEMENT (this “ AGREEMENT ”), dated as of October 6, 2009, is among ARCH CHEMICALS RECEIVABLES CORP., a Delaware corporation (the “ Seller ”), ARCH CHEMICALS, INC., a Virginia corporation, individually (“ Arch ”) and, as servicer (in such capacity, the “ Servicer ”, and together with the Seller, the “ Seller Parties ”), THREE PILLARS FUNDING LLC, a Delaware corporation (“ TPF ”), SUNTRUST ROBINSON HUMPHREY, INC. (F/K/A SUNTRUST CAPITAL MARKETS, INC.), a Tennessee corporation, as administrator (the “ Existing Administrator ”) MARKET STREET FUNDING LLC, a Delaware limited liability company (“ Market Street ”), and PNC BANK, NATIONAL ASSOCIATION, a national banking association (“ PNC ”), as successor administrator (the “ Successor Administrator ”), and acknowledged and agreed to by ARCH TREATMENT TECHNOLOGIES, INC., a Virginia corporation, ARCH WOOD PROTECTION, INC., a Delaware corporation, and ARCH PERSONAL CARE PRODUCTS, L.P., a New Jersey limited partnership (collectively, the “ Subsidiary Originators ”).

BACKGROUND

The Seller, TPF, the Servicer and the Existing Administrator are parties to that certain Receivables Purchase Agreement dated as of June 27, 2005 (as amended, restated, supplemented or otherwise modified prior to giving effect to any amendment on the date hereof, the “ Receivables Purchase Agreement ”). Capitalized terms used and not otherwise defined herein have the respective meaning assigned to such terms in the Receivables Purchase Agreement.

WHEREAS, TPF and the Existing Administrator each desires to transfer all of its right, title, interest and obligations in, to and under the Receivables Purchase Agreement and the Receivable Interest to Market Street and the Successor Administrator, as applicable, and each of Market Street and the Successor Administrator, as applicable, desires to purchase and assume all of the TPF’s and the Existing Administrator’s, as applicable, right, title, interest and obligations in, to and under the Receivables Purchase Agreement and the Receivable Interest and to become party to the Receivables Purchase Agreement.

NOW, THEREFORE, for good and valuable consideration the receipt and sufficiency of which is hereby acknowledged, the parties hereto hereby agree as follows:

SECTION 1. Representations and Warranties . The Seller desires that each of TPF and the Existing Administrator transfer, as applicable, all of its right, title, interest and obligations in, to and under the Receivable Purchase Agreement, the Receivable Interest, all related property and Invested Amount and that Market Street becomes a party to the Receivables Purchase Agreement and the Successor Administrator becomes the Administrator under the Receivables Purchase Agreement and upon the terms and subject to the conditions set forth herein. Each of TPF and the Existing Administrator agrees to transfer, as applicable, all of its right, title, interest and obligations in, to and under the Receivables Purchase Agreement, the Receivable Interest, all related property and the Invested Amount (the “ Transfer ”) on and as of October 6, 2009 (the

“ Closing Date ”). Market Street and the Successor Administrator, as applicable, agree to purchase and assume, all of TPF’s and the Existing Administrator’s, as applicable, right, title, interest and obligations in, to under the Receivables Purchase Agreement, the Receivable Interest, all related property and Invested Amount and Market Street agrees to become a party to the Receivables Purchase Agreement and the Successor Administrator agrees to become the Administrator under the Receivables Purchase Agreement. Market Street and the Successor Administrator shall not be a party to the Receivables Purchase Agreement for any purpose before the effectiveness of this Agreement, and immediately after the effectiveness of this Agreement and the Transfer, Market Street and the Successor Administrator and the other parties to the Receivables Purchase Agreement shall amend and restate the Receivables Purchase Agreement pursuant to the terms of that certain Amended and Restated Receivables Purchase Agreement, dated as of October 6, 2009 (the “ Amended and Restated Receivables Purchase Agreement ”).

Each of the Seller Parties hereby represents and warrants to Market Street and the Successor Administrator as of the date hereof, as follows:

(i) the representations and warranties of such Seller Party contained in Section 5.1 of the Receivables Purchase Agreement are true and correct in all material respects on and as the date of the Transfer as though made on and as of such date (except for representations and warranties which apply as to an earlier date, in which case such representations and warranties shall be true and correct as of such earlier date);

(ii) no event has occurred and is continuing, or would result from the Transfer, that constitutes a Amortization Event or Unmatured Amortization Event; and

(iii) the Facility Termination Date has not occurred.

SECTION 2. Transfer and Assumption of Purchases; Assignment of Receivable Interest .

(a) For effect on the Closing Date, TPF hereby transfers and assigns to Market Street, and Market Street hereby accepts and assumes TPF’s obligation to fund Purchases through issuance of Commercial Paper or a Liquidity Funding pursuant to the terms of the Receivables Purchase Agreement up to the Purchase Limit. Following such transfer to and assumption by Market Street, TPF shall no longer have an obligation to fund a Purchase pursuant to the terms of Receivables Purchase Agreement.

(b) Upon receipt of the amount described in Section 5(c) below on the Closing Date, TPF hereby sells and assigns to Market Street without recourse and without representation or warranty (except that it is the sole owner of and has good and marketable title to the portion of Receivable Interest being transferred hereunder, free of any Adverse Claim created by it), and Market Street hereby purchases and assumes TPF’s interest in and to the Receivable Interest, the Invested Amount related thereto and that portion of TPF’s other rights and obligations under the Receivable