Microsoft has said it will allow rival software companies access to license parts of the source code for its Windows operating system.
The EU has accused Microsoft of dragging its feet
The concession was made in response to a 2004 European Commission anti-trust ruling, which ordered the company to be more open to competitors' needs.
It came three weeks ahead of the EU's compliance deadline, which threatened fines of 2m euros (£1.4m; $2.4m) a day.
The commission said it was not sure the offer would help resolve the dispute.
Microsoft's legal chief, Brad Smith insisted "the source code is the ultimate documentation.
"It should have the answer to any questions that remain."
But competition commissioner Neelie Kroes disagreed.
"Normally speaking, the source code is not the ultimate documentation of anything," she said.
"[This is] precisely the reason why programmers are required to provide comprehensive documentation to go along with their source code."
Needle in a haystack
Microsoft said the code will help rivals make their software compatible with its own.
"We will ... license the Windows source code itself," said Mr Smith.
"Today we are putting our most valuable intellectual property on the table so we can put technical compliance issues to rest and move forward with a serious discussion about the substance of the case."
Rivals were not impressed.
"Even the most sophisticated software engineers would be lost," said Thomas Vinje, a lawyer who represents some Microsoft opponents, dismissing Microsoft's move as a PR stunt.
"They would dump millions of lines of code and finding what competitors need to interoperate would be like looking for a needle in a haystack."
The compliance deadline was set in December when the European Commission said that Microsoft's offer of 12,000 pages of documentation and 500 hours of free technical support was not adequate.
The landmark 2004 ruling said the world's biggest software company was guilty of abusing its position and hit Microsoft with a record 497m euros fine, telling it to open up its operating systems.
Microsoft said that the latest concession went "far beyond" the 2004 decision.
It maintains that it has tried to comply with the EU's demands, but says that Brussels keeps changing its guidelines.
The EU's second highest court, the European Court of First Instance, will hear Microsoft's appeal against the 2004 ruling in April.