UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of

the Securities Exchange Act of 1934

Filed by the Registrant ☐                Filed by a Party other than the Registrant ☒

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☐ Definitive Additional Materials

☒ Soliciting material Pursuant to §240.14a-12

Automatic Data Processing, Inc.

(Name of Registrant as Specified In Its Charter)

William A. Ackman

Veronica M. Hagen

V. Paul Unruh

Pershing Square Capital Management, L.P.

PS Management GP, LLC

Pershing Square, L.P.

Pershing Square II, L.P.

Pershing Square International, Ltd.

Pershing Square Holdings, Ltd.

Pershing Square VI Master, L.P.

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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On August 25, 2017, Pershing Square Capital Management, L.P. and certain of its affiliates posted the following material to www.ADPascending.com:

CERTAIN INFORMATION CONCERNING THE PARTICIPANTS

Pershing Square Capital Management, L.P. (“Pershing Square”) and certain of its affiliated funds have filed a preliminary proxy statement and an accompanying proxy card with the Securities and Exchange Commission (the “SEC”) to be used to solicit proxies in connection with the upcoming annual meeting of stockholders (the “Annual Meeting”) of Automatic Data Processing, Inc. (the “Company”) and the election of a slate of director nominees at the Annual Meeting (the “Solicitation”). Prior to the Annual Meeting, Pershing Square and certain of its affiliated funds intend to file with the SEC, and furnish to stockholders of the Company, a definitive proxy statement and accompany proxy card.

PERSHING SQUARE STRONGLY ADVISES ALL STOCKHOLDERS OF THE COMPANY TO READ THE PRELIMINARY PROXY STATEMENT, THE DEFINITIVE PROXY STATEMENT, ANY AMENDMENTS OR SUPPLEMENTS TO SUCH PROXY STATEMENT AND OTHER PROXY MATERIALS RELATED TO THE SOLICITATION WHEN THEY BECOME AVAILABLE, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION, INCLUDING INFORMATION RELATING TO THE PERSONS WHO MAY BE DEEMED PARTICIPANTS IN THE SOLICITATION. SUCH PROXY MATERIALS WILL BE AVAILABLE AT NO CHARGE ON THE SEC’S WEB SITE AT HTTP://WWW.SEC.GOV. IN ADDITION, THE PARTICIPANTS IN THIS PROXY SOLICITATION WILL PROVIDE COPIES OF THE PROXY STATEMENT AND OTHER RELEVANT DOCUMENTS WITHOUT CHARGE, WHEN AVAILABLE, UPON REQUEST. REQUESTS FOR COPIES SHOULD BE DIRECTED TO THE PARTICIPANTS’ PROXY SOLICITOR, D.F. KING & CO., INC., 48 WALL STREET, 22ND FLOOR, NEW YORK, NEW YORK 10005 (CALL COLLECT: (212) 269-5550; CALL TOLL FREE: (866) 342-1635) OR EMAIL: ADP@DFKING.COM.

William A. Ackman, Veronica M. Hagen, V. Paul Unruh, Pershing Square, PS Management GP, LLC, Pershing Square, L.P., Pershing Square II, L.P., Pershing Square International, Ltd., Pershing Square Holdings, Ltd. and Pershing Square VI Master, L.P. may be deemed “participants” under SEC rules in the Solicitation. William A. Ackman, Pershing Square and PS Management may be deemed to beneficially own the equity securities of the Company described in Pershing Square’s statement on Schedule 13D initially filed with the SEC on August 7, 2017 (the “Schedule 13D”), as it may be amended from time to time. Except as described in the Schedule 13D, none of the individuals listed above has a direct or indirect interest, by security holdings or otherwise, in the Company or the matters to be acted upon, if any, in connection with the Annual Meeting.