A US court of appeals yesterday dented Sun Microsystems’ bid to show that Microsoft illegally "polluted" the Java programming language in an attempt to undermine Java's success.

In an opinion released today, the appeals court agreed with Microsoft's contention that a lower court made a mistake when it granted Sun a preliminary injunction in the case. The injunction, awarded in November 1998, prevented Microsoft from using a version of Java in its software that wasn't fully compatible with Sun's.

But the appeals court also allowed that Sun has amassed "significant evidence" in its case against Microsoft, and upheld the lower court's opinion that Sun is likely to prevail in the case based on its merits. What the appeals court is looking for is clarification from the lower court as to why it awarded Sun the injunction.

Part of the disagreement between Sun and Microsoft centres on whether Sun's lawsuit was for copyright infringement, as Sun contested, or whether it boiled down to a contract dispute, as Microsoft argued.

Judge Ronald Whyte of the California District Court in San Jose determined it was an infringement case, which in legal terms entitled Sun to a claim of irreparable harm. It was partly on that basis of irreparable harm that Whyte granted Sun the preliminary injunction.

The appeals court yesterday said Whyte never fully explained his reasons for treating the case as an infringement lawsuit, and on that basis annulled his decision.

"We agree with Sun that significant evidence supports the district court's holding that Sun is likely to prevail on its interpretation of the language of the agreement and to prove that Microsoft's conduct violated it," Judge Mary Schroeder wrote in the appeal's court's majority opinion.

"We agree with Microsoft, however, that the district court should not have invoked the presumption of irreparable harm applicable to copyright infringement claims," without clearly spelling out its reasons, Schroeder wrote.

Sun also accused Microsoft of unfair competition under California law. The district court entered an injunction on that claim solely on the basis of Microsoft's past conduct, the appeals court said.

"Microsoft correctly contends that under California law an injunction must be based on the prospect of future conduct. We therefore also vacate the injunction insofar as it relates to the unfair competition claim and remand for consideration of that issue."

Sun did not immediately comment on the decision.