In the battle between two pending online music subscription firms to sign up major partners, Microsoft Corp. Thursday sided with Sony Music Entertainment's and Universal Music Group's pressplay to erect a co-branded site. The deal, in a way, again pits RealNetworks' RealPlayer versus Windows Media.

Specific terms of the agreement were not disclosed, but MSN Music will serve as an affiliate of pressplay's online music subscription service, which is due this summer. The shared site will provide the approximately five million MSN Music users with a large catalog of music from artists in a number of genres.

The alliance is a further boon for pressplay because it will benefit from offering the thousands of songs in the popular Windows Media format and Windows Media digital rights management (DRM) technology for content protection. MSN Music roared out of the gate in April and has since added new features to tempt music fans.

On the flipside, MSN hopes its subscribers will become enamored of pressplay, which has its own huge music catalog courtesy of Sony and Vivendi, and is formatted to allow listeners to search for the music they want to listen to on demand and to share personalized playlists.

"This agreement is a natural extension of the popular services we offer consumers today on MSN Music," said Yusuf Mehdi, vice president of MSN.

It is also a natural extension of Microsoft's digital media rivalry with RealNetworks Inc., which along with AOL Time Warner, Bertelsmann AG and EMI Group, has teamed to forge MusicNet, a service similar to pressplay, also due later this summer.