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Margaret K. Pfeiffer

Senior Counsel

Margaret K. Pfeiffer

Senior Counsel
Washington, D.C. +1-202-956-7540+1-202-956-7540 +1-202-293-6330+1-202-293-6330
[email protected]

Margaret Pfeiffer was a partner of Sullivan & Cromwell from 1982 to 2008, resident in the Washington, D.C. office. She became senior counsel in 2011. Ms. Pfeiffer’s practice includes the representation of U.S. and non-U.S. clients of the Firm in state and federal courts throughout the country.

Among the areas on which her practice focuses is patent litigation. Ms. Pfeiffer has represented both patentees and accused infringers in actions before the federal courts and the International Trade Commission (“ITC”). Recent matters include the successful representation of Philips Electronics N.V. in enforcing patents related to recordable compact discs, in both the ITC and New York federal court, and defending VeriFone, Inc. in an ITC investigation and related litigation in California federal court, in which VeriFone was accused of infringing patents relating to credit verification point-of-sale devices. Ms. Pfeiffer has been selected by her peers for inclusion in the Best Lawyers in America for Patent Litigation (2008-2022).

As part of her practice relating to advising and representing clients in regard to investigations by government agencies, Ms. Pfeiffer has long experience with civil and criminal enforcement of U. S. economic sanctions and dealing with the Department of Justice and the U.S. Securities and Exchange Commission (“SEC”) in regard to proceedings under the Foreign Corrupt Practices Act. She has also long advised and represented clients in matters involving restrictions on non-U.S. investment in the United States, including dealing with the Committee on Foreign Investment in the United States.

Ms. Pfeiffer is also experienced in antitrust litigation and in merger-related investigations and dealings with the Antitrust Division of the Department of Justice and the Federal Trade Commission (“FTC”), and provides antitrust advice to a range of clients, including Koninklijke Philips Electronics N.V. and Nippon Steel Corporation, on an ongoing basis.

Ms. Pfeiffer oversees the pro bono practice in the Washington, D.C. office.

Professional Activities and Community Involvement

  • Board Member, National Law Center on Homelessness and Poverty
  • Vice Chair, Shakespeare Theatre Lawyers Committee


  • Nippon Steel/Kawada Bridge Inc. v. Tacoma Narrows Constructors and Samsung Heavy Industries Co. Ltd. – Ms. Pfeiffer represents the Nippon Steel/Kawada Bridge joint venture in a contract dispute arising out of a subcontract to provide components and engineering services for a suspension bridge being built across the Tacoma Narrows in Washington State. The case is pending in state court in Olympia, Washington
  • SEC, U.S. Attorney’s Office and U.S. Dept. of Justice investigation – In ongoing criminal and civil investigations involving alleged violations of the Foreign Corrupt Practices Act, Ms. Pfeiffer is representing a non-U.S. oil company
  • Valdez v. State of Oklahoma – At the request of the government of Mexico, Ms. Pfeiffer represented, pro bono, a Mexican national on death row in Oklahoma, who was denied his rights under a treaty to which both Mexico and the U.S. are parties. She was successful in obtaining a grant of habeas corpus from the Oklahoma Court of Criminal Appeals and in negotiating a sentence of life imprisonment upon remand of the case (2004)