Office Open XML (also informally known as OOXML)[5] is a zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents.

[6] Microsoft Office 2013 and later fully support ISO/IEC 29500 Strict,[7] but do not use it as the default file format because of backwards compatibility concerns.

Microsoft announced in November 2005 that it would co-sponsor standardization of the new version of their XML-based formats through Ecma International as "Office Open XML".

[17] A technically equivalent set of texts is published by Ecma as ECMA-376 Office Open XML File Formats—2nd edition (December 2008); they can be downloaded from their website.

"[19] The same InfoWorld article reported that IBM (which supports the ODF format) threatened to leave standards bodies that it said allow dominant corporations like Microsoft to wield undue influence.

The covenant received a mixed reception, with some like the Groklaw blog criticizing it,[24] and others such as Lawrence Rosen, (an attorney and lecturer at Stanford Law School), endorsing it.

[29] Ecma International asserted that, "The OSP enables both open source and commercial software to implement [the specification]".

[43] Microsoft Office 2013 and later fully support ISO/IEC 29500 Strict,[7] but do not use it as the default file format because of backwards compatibility concerns.