Lawyers representing both sides in the appeal of the European Commission's 2004 antitrust ruling against Microsoft were left fumbling Thursday under cross-examination by one of the 13 judges hearing arguments.

Microsoft's appeal of the ruling is being heard in the Court of First Instance, Europe's second-highest court. Initially turning to Microsoft attorneys, Judge John Cooke asked them about the information being sought by the Commission to ensure that server software made by rival companies can work in a stack with Microsoft's Windows Server software.

"The suggestion by the Commission is that Microsoft is exaggerating the extent of the Commission remedy. Is this Microsoft's stance?" Cooke asked. His question temporarily left Microsoft attorney Ian Forrester speechless. Forrester, who is Microsoft's top advocate on the issue of interoperability, has so far in the appeals hearing proved himself to be particularly eloquent, so his silence was all the more striking.

Cooke came to Forrester's aid, rephrasing the question: "Are you offering to disclose algorithms in order to show that what the Commission is asking is impossible?" Forrester answered affirmatively, adding swiftly that he denies that Microsoft has exaggerated the Commission's demands.

Among the 13 judges, Cooke has proved to be, by far, the most knowledgeable about technology. After questioning Forrester, he turned to the Commission, questioning the assertion made Wednesday that Microsoft's trade secrets are of little value.

"These are not just trivial trade secrets. This is hugely valuable information developed by a major corporation taking hundreds of man hours to develop," Cooke said, adding that he found it hard to follow the Commission's logic in defense of its 2004 ruling.

It was then Anthony Whelan's turn to be rendered speechless. Whelan, one of the Commission's attorneys on interoperability, composed himself after a few seconds and agreed that the information the Commission seeks is strategically important.

Cooke's cross-examination does not necessarily reveal his thinking, representatives from both sides said after the court adjourned for lunch.

"The judge was just being provocative," said Thomas Vinje, a lawyer representing the European Committee for Interoperable Systems, a trade group and one of the Commission's key allies in the case.

Microsoft's general counsel for Europe agreed. "It's not possible to predict from the questions what the outcome will be," said Horacio Gutierrez, adding that Cooke in particular has shown "a tremendous grasp of the economic, technical and legal arguments."