* Exxon ordered to pay 5.9 pct interest on 1996 judgment
* Company ordered to pay own legal fees for appeal
* Court affirms amount of punitive damages at $507.5 mln (Adds comments from plaintiffs' attorney, no comment from Exxon, background)
LOS ANGELES, June 15 (Reuters) - A federal appeals court on Monday ordered Exxon Mobil Corp (XOM.N) to pay 5.9 percent interest back to 1996 on a judgment in the Valdez oil spill off Alaska's coast, and to pay its own legal fees for the appeal.
In an opinion on Monday, the Ninth Circuit Court of Appeals also affirmed the amount of punitive damages at $507.5 million that Exxon must pay related to the 1989 spill.
Exxon had previously agreed to pay that amount in compensatory damages to fishermen, Alaska natives, business owners and others who bore the economic brunt of the worst oil spill in U.S. history.
A jury had awarded the litigants $5 billion in punitive damages in 1996, but the U.S. Supreme Court ruled last year that the maximum ratio of punitive to compensatory damages was 1:1 under maritime law.
Exxon already has paid the $507.5 million in compensatory damages and $383 million toward punitive damages.
Jeffrey Fisher, an attorney for the plaintiffs, said Monday's opinion added about $500 million in interest to the punitive damages award.
According to the opinion, Exxon had asked the court to order the plaintiffs to pay all or at least 90 percent of Exxon's legal fees for the appeal. The opinion said those fees amounted to nearly $70 million.
In a split decision, both parties were ordered to pay their own legal fees.
The appeals court judge who dissented on the legal fees noted in his opinion that to insure the original $5 billion punitive damages verdict, Exxon had to pay $60.6 million to several banks.
Three judges were on the appeals panel.
An Exxon spokesman said the company was reviewing the opinion and had no immediate comment.
The Exxon Valdez supertanker ran aground in Price William Sound, leaking 11 million gallons of crude oil and blackening 1,200 miles of Alaska's coastline. The clean-up alone cost around $2.5 billion.
Fisher said the ruling marks "the last real piece of the case" except for some "minor side issues."
"We think they ought to pay now," he said. The case is In Re: The Exxon Valdez, No. 04-35182, U.S. Court of Appeals for the Ninth Circuit. (Reporting by Gina Keating in Los Angeles; Editing by Gary Hill, Toni Reinhold)