The deal, announced on Tuesday, has been agreed to in principle and is expected to be completed within the next few months.
"UIQ offers excellent technical flexibility, enabling us to provide compelling features such as push e-mail, Internet browsing, end-user personalization and enhanced music applications," said Mats Lindoff, Sony Ericsson's chief technology officer.
"By acquiring UIQ," he said, "we will further invest and exploit the full potential of UIQ on Symbian's (operating system) for phone vendors, mobile operators, developers and consumers."
Symbian's chief executive,, said the move would "strengthen the capabilities and services available for phones based on UIQ and Symbian OS while allowing us to focus on the core product ."
Sony Ericsson is already by far the most significant licensee of the operating system, with UIQ version 3.0 being used in the recent P990, M600 and W950 handsets.
The move is the latest chapter in a complex relationship between the three organizations. Mobile Applications Lab, the company that went on to become UIQ, was originally set up by Ericsson Mobile Communications in 1998. UIQ became part of Symbian in 1999.
"Most of the investors in Symbian must have been fairly down on subsidizing Sony Ericsson's software development," analyst Dean Bubley of Disruptive Analysis said. "Motorola is not using it anymore--essentially, the Sony Ericsson smart-phone platform was being paid for by Symbian."
Bubley linked the announcement to Monday's news that influential operator Vodafone was pushing for a reduction in the number of handset operating systems with the hope of concentrating on Windows Mobile, Symbian Series 60 and Linux within the next five years.
"Given Sony Ericsson's resurgence in the last year or so, there is an argument that if it is serious about this, it will take UIQ and make it into (its) equivalent of Series 60," said Bubley, adding that this strategy would make it easier for Sony Ericsson to negotiate a place for its own preferred operating system at the operators' high table.
David Meyer of ZDNet UK reported from London.