In a vaguely worded announcement, Sunoco said that it immediately withdrew from the oil-bidding process after becoming aware of "improper conduct." It conducted an internal investigation and notified the Justice Department. Sunoco said it was cooperating with authorities.

Sunoco did not name the employees or the entities involved in the trading. But industry sources said the bidding apparently involved the Nigerian National Petroleum Corp. Sunoco is a major buyer of West African crude to supply its three Philadelphia refineries. It also owns a refinery in Ohio.

The announcement came after several trade publications reported in June that three senior Sunoco executives in Philadelphia and a trader in London were no longer at their desks amid speculation about an investigation into trading practices.

The Oil Price Information Service of Wall, N.J., quoted an internal Sunoco memorandum in June that said the four employees were "on a confidential special assignment."

It identified the four as Tony Hepworth, vice president for crude and raw material trading and supply, Joe Colella, vice president of product trading sales and supply, Meredith Earley, a manager in crude supply and trading, and Anwar Elahi, a London trader.

A person who answered the telephone at Hepworth's home in West Chester last night declined to comment. Earley, reached at her Bryn Mawr home, referred questions to her attorney.

"Meredith is proud of her service at Sunoco," said Gregory Paw, who represents the 22-year employee. "She is cooperating fully with Sunoco's investigation."

Thomas Golembeski, Sunoco spokesman, said a crude-oil processing transaction typically involves a refiner agreeing to process petroleum in exchange for a fee and a percentage of the product. He said Sunoco ordinarily did not engage in such deals.

"Sunoco does not currently believe the matter will result in any material liabilities or revision of financial statements," the company said in a statement. "Sunoco conducts its business with honesty and integrity."

The announcement came after markets closed. Sunoco's stock closed up 2.4 percent at $22.30 a share.