New phone signals break from Symbian.
Sony Ericsson has signed a development deal with Microsoft to produce a phone using the Windows Mobile operating system.
The deal was unveiled at Mobile World Congress in Barcelona, where Sony Ericsson announced the launch of seven new handsets.
One of the new models, the Windows Mobile Xperia X1, has a 3in touch screen similar to Apple's iPhone and a built-in sliding keyboard.
"Our vision for the X1 is to deliver a seamless blend of mobile web communication and multimedia entertainment within a distinctive design," said Rikko Sakaguchi, head of portfolio and propositions at Sony Ericsson.
"Xperia is our promise to deliver the premium experience anytime, anywhere, anything and with anyone."
Sony Ericsson said that the handset would be released in the later part of 2008 but would not give details on pricing.
This is the first time that Sony Ericsson has not used the Symbian operating system, in which it is a shareholder. A spokesman said that the choice of operating system was less important than the user experience.