The companies are set to release on Thursday a limited beta test of a service that will enable users of Windows Live Messenger (the next generation of MSN Messenger) and Yahoo Messenger with Voice to connect with each other.
The move creates a global community of nearly 350 million accounts, the companies said. The beta service is being launched globally in 15 localized languages.
"It's the first-ever bridge between two global instant messaging services," Blake Irving, corporate vice president in Microsoft's Windows Live Platform division, said Wednesday.
The service will allow people to sign into both services using one user ID, and to see the status of connections of friends from both networks.
"The messenger friend list is your heart of your social network in many ways," said Brad Garlinghouse, senior vice president of communications, community and front doors at Yahoo. "We expect a rapid ramp with millions of users in the coming weeks."
Connecting the networks took months of testing, Irving said. "This was a very difficult technical problem to solve," he said.
The companies announced plans last October, in what analysts said was a shot to market leader AOL Instant Messenger and a defensive jab at Google Talk.
AOL representatives could not be reached for comment.
A Google representative provided an e-mail statement that said: "From the very beginning, the Google Talk service was built to support interoperability with hundreds of other communications service providers. Any service provider that supports industry standard protocols can federate with us today, and many have...We don't have further details to share on our future plans in this area at this time."
Under, in which Google paid $1 billion for a 5 percent stake, the two companies are working to make their respective instant messaging services interoperate.
Neither Microsoft nor Yahoo would comment on the status of any discussions with AOL about interoperating.
"We certainly welcome seeing other industry players come to the table," Yahoo's Garlinghouse said. "We're blazing a trail for how interoperability is done."
David Card, an analyst at JupiterResearch, said AOL would not move to interoperate with others until customers demanded it.
"AOL doesn't need to let the other guys in as long as the user base is still healthy," he said. "I think they will let people in, eventually. This will help pressure them a little bit more."