The interoperability programme is currently available as a limited beta in a series of countries including the UK and US. The final offerings will be released in the coming months, the two companies said in a joint press release.
Users can sign up for the beta on the Microsoft and Yahoo webpages. The sign-up information was not available as of press time. A Microsoft spokesman said that it would be posted later on Thursday.
The beta period is intended to allow for scalability testing of the new service, according to the spokesman.
"This first-of-its-kind interoperability between consumer IM leaders Microsoft and Yahoo gives our customers tremendous control, convenience and freedom in their web communication experiences with Windows Live," said Blake Irving, corporate vice president for the Windows Live Platform at Microsoft.
"We are proud to deliver this latest advancement in IM services that empower people to communicate with virtually whomever they want, wherever they want and whenever they want."
Microsoft and Yahoo promised last October to provide interoperability between their respective IM services. The project was originally scheduled for completion by the end of last month.
The providers claimed that their combined services make for the world's largest IM network of 350 million subscribers.
Data from comScore Media Metrics from last May pegs Microsoft at 204m subscribers worldwide, followed by Yahoo with 78m unique visitors. The AOL Instant Messenger claims the number three spot with 34m unique visitors.
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