An investing group led by Mesa Petroleum Co. Chairman T. Boone Pickens Jr. said yesterday it would sell its 5.8 percent holding in Phillips Petroleum Co. back to the company for $53 a share -- taking an $89 million profit in the process.

The sale, which fulfills a previously announced agreement between Phillips and the Pickens investors, ties up one of the remaining loose ends of the recent pursuit of Phillips by the Pickens group and the subsequent challenge to the company by another group led by New York investor Carl C. Icahn.

Phillips won its freedom earlier this week by agreeing to swap $4.5 billion worth of debt for about half its stock, an offer Icahn indicated he would accept. Icahn, who, like Pickens, also received $25 million for the expenses of his challenge to the company, then agreed to drop his $8.5 billion offer to buy out Phillips.

Pickens said his group took the $53 cash offer from Phillips rather than accepting the debt securities because it believed that it would have gotten the equivalent of $53 either way, and by taking the cash it ensures that the overall payout to other Phillips shareholders will increase slightly. Phillips has estimated that its offer of the securities and other inducements will be worth about $56 or $57 to shareholders, but Pickens spokesman David Batchelder said that with the group's sale of stock reducing the total number of Phillips shares outstanding, the company's offer should be worth about $55 a share.

"We think that everybody here has got at least $53. We'll get $53 by selling back, and if the plan was worth $53 with Mesa tendering, it will be worth at least that by Mesa selling its shares back to the company," Batchelder said. He said the transaction will be completed within five days.

The Pickens group's agreement to sell its 8.9 million shares of Phillips stock back to the company was part of a peace treaty that ended Pickens' pursuit of the company last December. As another part of that pact, Phillips asked shareholders to approve the recapitalization. Icahn led the opposition to that plan, and Phillips shareholders failed to approve it, leading the company to make its sweetened offer, and peace with Icahn