Microsoft's appeal against a $520 million patent infringement award presented to Eolas will be heard on Thursday.

In August last year, a jury found that Microsoft's Internet Explorer had infringed on Eolas and the University of California's patent that allows interactive content to be embedded in a website - a common practice on the Internet. In January this year, a judge upheld the ruling.

However, it triggered an outcry from experts, who argued that the patent should be invalid because of prior art, or examples of the technology's use before the patent was issued. Tim Berners-Lee, director of the World Wide Web Consortium (W3C), urged the US Under Secretary of Commerce for Intellectual Property to invalidate the patent.

The US Patent and Trademark Office (USPTO) agreed to re-examine the patent and in March rejected it. The University of California, holder of the patent, is appealing that decision.

With the process at the USPTO still underway, the US Court of Appeals is scheduled to hear the Eolas case. Microsoft plans to question the validity of the patent, spokesman Jim Desler said. "One of our main points is going to be over the question of the validity of the patent and that the jury was prevented from hearing and considering evidence related to important prior art during the course of the trial," Desler said.

The University of California and Eolas are confident of their case, said Trey Davis, director of special projects and new media for the university. "The patent was appropriately issued to the university and consistent with the jury's finding that Microsoft infringed upon the patent, Microsoft owes us in excess of half a billion dollars," Davis said.

The University of California was issued the 5,838,906 patent in November 1998. The technology covered in the patent was developed by researcher Michael Doyle, who went on to found Eolas. It describes in part "a system allowing a user of a browser program... to access and execute an embedded program object." Eolas has exclusive rights to use and license the patent.

The hearing at the Washington is expected to last no longer than one day. It is not known when the court will rule.