The European Commission, the EU's executive, has threatened Microsoft with the heavy penalty for failing to give information to competitors, as required under a 2004 antitrust decision which found that the dominant company competed illegally.
This week, Microsoft Chief Counsel Brad Smith called a news conference in Brussels to announce that the firm had found a novel way to deal with the problem: letting competitors see some of its source code, the blueprint of its software.
"The Windows source code is the ultimate documentation of Windows server technologies," he said. "With this step our goal is to resolve all questions about the sufficiency of our technical documentation."
Kroes took a different view.
"Normally speaking, the source code is not the ultimate documentation of anything, which is precisely the reason why programmers are required to provide comprehensive documentation to go along with their source code," she said.
Kroes said she had not expected the Microsoft announcement.
"It was a surprise that they decided to disclose the source code," she said in a telephone interview from the World Economic Forum at Davos, Switzerland.
Kroes's spokesman in Brussels said that the Commission had received nothing beyond Microsoft's news release and a letter to Kroes from Microsoft Chief Executive Steve Ballmer.
"Until such time as we receive the details in writing, we cannot begin our analysis," the spokesman, Jonathan Todd, said.