The format Microsoft Corp.'s Office 2007 programs use to save documents was approved as an international standard Tuesday, a step the company touted as proof it is willing to make once-proprietary technology work openly with competing programs.

But the International Standards Organization vote didn't quiet some opponents, who argued that the Office Open XML standard still locks out competitors and gives Microsoft customers no choice but to keep buying its programs forever.

The decision was made public on the Web site of a European standards organization, Ecma International, on Tuesday. ISO is expected to formally announce the vote Wednesday.

Years before Microsoft started selling Office 2007, its plan to switch to a new way of saving Word, Excel and PowerPoint files sparked concerns that future changes made by the software maker could render government or corporate archives unreadable, especially by non-Microsoft programs.

In 2005, Massachusetts went so far as to direct state government offices to save documents as OpenDocument Format files, already an ISO standard whose inner workings were freely available to Microsoft and competing software makers alike.

The responsibility for maintaining standards falls to groups like ISO, not the companies that developed them.

Massachusetts reversed its stance after Microsoft promised to turn over responsibility for the OOXML format to Ecma International, a European standards body. In January 2007, Ecma submitted OOXML to ISO, a broader international organization, for a sped-up approval process.

Both Microsoft's OOXML and ODF - backed by Sun Microsystems, IBM and others - make it easier for computer programs to read and understand the contents of a file. That helps ensure older files will still be readable even if the program used to create them has changed significantly.

Before Microsoft turned OOXML over to Ecma, the software maker had sole control over the technology and no obligation to make it compatible with rivals' products. Critics argued that Microsoft essentially gave customers no choice but to stick with Office if they wanted documents to be readable in the future. They also worried that Microsoft would drop support for OOXML someday, leaving customers with no way to see old files.

Making OOXML a standard is meant to address both problems, and ensure that competing programs can read and save OOXML files today and read them far into the future, even if Microsoft moves on.

"The input from technical experts, customers and governments around the world has greatly improved the Open XML specification and will make it even more useful to developers and customers," said Tom Robertson, general manager of interoperability and standards at Microsoft, in a statement. "We are committed to supporting this specification in our products, and we will continue to work with standards bodies, governments and the industry to promote greater interoperability and innovation."

In an interview last week, Robertson said Microsoft and Ecma addressed almost all the issues raised during the ISO process, but some critics remain unsatisfied.

Even though Norway's standards body voted for OOXML, Steve Pepper, a member of the organization, said Microsoft used strong-arm tactics to win approval.

"This vote is a tragedy for standardization," Pepper told The Associated Press in Switzerland.

And Marino Marcich, head of the OpenDocument Format Alliance, said that despite promised changes to the specification, "OOXML doesn't play well with the products of other software companies" like IBM and Sun. He claimed that Microsoft failed to fully document all the features in the format and that some of the code was proprietary, leaving other companies vulnerable to copyright lawsuits.

What's more, Marcich said, Microsoft can still use a version of OOXML in Office 2007 that doesn't exactly match the one approved by ISO.

In a statement, Robertson said Microsoft is "committed to supporting this specification in our products."