The much-vaunted pairing comes a bit later than the two companies had originally hoped.
The project was delayed because Microsoft wanted to make sure the systems would work well with both companies' millions of users, Microsoft executive Blake Irving said Wednesday.
"It's not the technical difficulty of the service itself. It's the technical difficulty of the scale that we're trying reach," said Irving, a corporate vice president for the Windows Live Platform.
Microsoft and Yahoo announced plans last October to let people using either company's instant messaging software send lightning-fast bits of text to each other. The system was supposed to be in place by June.
Right now, Microsoft, Yahoo and Time Warner Inc.'s (TWX) AOL all have separate instant messaging systems that don't easily work together, creating hassles for people who want to communicate with friends or colleagues using other programs.
Microsoft does sell a product that lets business users send and receive messages from the competing systems, but that's not available for people who just have the company's free offering.
The pairing of Microsoft and Yahoo — who compete aggressively in other areas — has been seen as a way to better take on U.S. market leader AOL and perhaps even form a defense against mutual rival Google Inc. (GOOG).
Although Google is still a minor player in instant messaging, the two companies have reason to fear the Internet search leader's ever-expanding reach.
The test of the companies' interoperability plans will initially be available only to limited numbers of people who have the latest version of either company's instant messaging products: Windows Live Messenger and Yahoo Messenger with Voice. The functionality is expected to be broadly available by the end of the year.
The development comes as instant messaging has expanded to include even more communications options, such as voice and video. That's made the increasingly sophisticated systems a more integral part of people's work and home lives.
For now, the Yahoo-Microsoft partnership will only work for sending text back and forth, although the companies said there are plans to eventually add voice capabilities.
"We want to get it right, not just get it out," said Brad Garlinghouse, a Yahoo senior vice president. "We will be implementing voice. We don't have a date to share with you at this point."
The executives said there are no current plans to add video capabilities between the two systems, but it could be a possibility later on.