Microsoft and Yahoo have agreed to make their instant-messaging services compatible with each other, and the deal is expected to be announced today (Wednesday), reports the Associated Press, citing unnamed sources. Until now, none of the leading consumer IM systems have communicated directly with each other (enterprise versions have), although Microsoft has reportedly been in talks with AOL over possible partnerships with MSN.

The Yahoo Messenger and MSN Messenger combined user base would become nearly as large as that of market leader AOL Instant Messenger (AIM), which has 51.5 million unique U.S. users, according to Nielsen/NetRatings; MSN Messenger has 27.3 million, and Yahoo Messenger has 21.9 million.

Google last month launched its own service, Google Talk, which includes voice chat. IM has become important as a business communication tool, with service providers adding video and internet telephone functions to them.