The exchange of both text and voice messages is being considered, although the source stressed that details of the pact are still being finalized. The two companies are planning to announce the deal on Wednesday, the source said.
A Microsoft representative declined to comment on the matter. A Yahoo representative was not immediately available for comment.
On Wednesday morning, the two companies announced a press conference for 8 a.m. PDT to discuss "a significant global communications agreement for customers."
The three major IM providers--Yahoo, Microsoft and AOL--have talked about interoperability for some time, but there.
For some months now, workers at businesses running Microsoft'shave been able to exchange text messages across multiple instant messaging programs. However, consumers have had to manage multiple accounts in order to use more than one of the big three services: AOL Instant Messenger, Yahoo Messenger and MSN Messenger. Third-party programs like Trillian have allowed users to connect to multiple services within a single program.
Details of the Yahoo-Microsoft agreement were reported earlier Tuesday by the Wall Street Journal.
CNET News.com's Alorie Gilbert contributed to this report.