EX-99.3

Offer to Exchange up to $720,000,000 Principal Amount of 9.75% Senior Notes due 2023 for a Like Principal Amount of 9.75% Senior Notes due 2023 which are registered under the Securities Act of 1933 (the “2023 Notes Exchange Offer”)

Offer to Exchange up to $500,000,000 Principal Amount of 10.00% Senior Notes due 2025 for a Like Principal Amount of 10.00% Senior Notes due 2025 which are registered under the Securities Act of 1933 (the “2025 Notes Exchange Offer and, together with the 2023 Notes Exchange Offer, the “exchange offers” and each an “exchange offer”)

Blue Cube Spinco Inc., a Delaware corporation (the “Issuer”), and a wholly owned subsidiary of Olin Corporation (the “Parent Guarantor”), hereby offers to exchange, upon and subject to the terms and conditions set forth in the prospectus dated , 2016 (the “Prospectus”) and the related letter of transmittal (the “Letter of Transmittal”), (i) up to $720,000,000 aggregate principal amount of its outstanding, unregistered 9.75% Senior Notes due 2023 (the “Original 2023 Notes”) for an equivalent amount of registered 9.75% Senior Notes due 2023 (the “Exchange 2023 Notes”) and (ii) up to $500,000,000 aggregate principal amount of its outstanding, unregistered 10.00% Senior Notes due 2025 (the “Original 2025 Notes” and, together with the Original 2023 Notes, the “Original Notes” and each an “Original Note”) for an equivalent amount of registered 10.00% Senior Notes due 2025 (the “Exchange 2025 Notes” and, together with the Exchange 2023 Notes, the “Exchange Notes” and each an “Exchange Note”). The Original Notes and the Exchange Notes are sometimes referred to in this letter together as the “Notes.” The Original Notes are, and the Exchange Notes will be, fully and unconditionally guaranteed on a senior unsecured basis by the Parent Guarantor. Capitalized terms not defined herein shall have the meaning ascribed to them in the Prospectus.

The exchange offers are intended to satisfy certain obligations of the Issuer and the Parent Guarantor contained in the Registration Rights Agreement, dated October 5, 2015, by and among the Issuer, the Parent Guarantor and J.P. Morgan Securities and Wells Fargo Securities, LLC, for themselves and as representatives of the Initial Purchasers (as defined therein), relating to the Original 2023 Notes and the Original 2025 Notes. As set forth in the Prospectus, the terms of the Exchange Notes are identical in all material respects to the terms of the corresponding series of the Original Notes, except that the Exchange Notes are registered under the Securities Act of 1933, as amended, and the transfer restrictions, registration rights and payment of additional interest in case of non-registration applicable to the Original Notes do not apply to the Exchange Notes.

Please contact your clients for whom you hold Original Notes regarding the exchange offers. For your information and for forwarding to your clients for whom you hold Original Notes registered in your name or in the name of your nominee, or who hold Original Notes registered in their own names, the following documents are enclosed:

Your prompt action is requested. The exchange offers will expire at 11:59 p.m., New York City time, on , 2016, unless extended (such date and time, the “Expiration Date”). Any Original Notes tendered pursuant to the exchange offers may be withdrawn at any time prior to the Expiration Date.

The Issuer and the Parent Guarantor will, upon request, reimburse brokers, dealers, commercial banks and trust companies for reasonable and necessary costs and expenses incurred by them in forwarding the Prospectus and related documents to the beneficial owners of the Original Notes held by them as nominee or in a fiduciary capacity. The Issuer and the Parent Guarantor have not retained any dealer-manager in connection with the exchange offers and will not make any payment to brokers, dealers or others soliciting acceptances of the exchange offers.

To participate in the exchange offers, a duly executed and properly completed Letter of Transmittal (or facsimile thereof), with any required signature guarantees, and any other documents required by the Letter of Transmittal or an Agent’s Message (as defined in the Letter of Transmittal) stating that the tendering holder has expressly acknowledged receipt of, and agrees to be bound by and held accountable under, the Letter of Transmittal, must be sent to the Exchange Agent and certificates representing the Original Notes (or confirmation of book-entry transfer of such Original Notes into the Exchange Agent’s account at The Depository Trust Issuer) must be delivered to the Exchange Agent, all in accordance with the instructions set forth in the Letter of Transmittal and the Prospectus.

Any inquiries you may have with respect to the exchange offers or requests for additional copies of the enclosed materials should be directed to the Exchange Agent at its address and telephone number set forth on the front of the Letter of Transmittal.

NOTHING HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU OR ANY OTHER PERSON AS AN AGENT OF THE ISSUER OR THE EXCHANGE AGENT, OR AUTHORIZE YOU OR ANY OTHER PERSON TO USE ANY DOCUMENT OR MAKE ANY STATEMENTS ON BEHALF OF ANY OF THEM WITH RESPECT TO THE EXCHANGE OFFERS, EXCEPT FOR STATEMENTS EXPRESSLY MADE IN THE PROSPECTUS OR THE LETTER OF TRANSMITTAL.