Integration of software applications can be achieved on different levels: the data level, the business logic level, and the user interface level. Integration on the user interface level means assembling the user interfaces of existing applications in a framework allowing seamless, unified interaction with those applications. While integration on the user interface level is desirable both from an software engineering as well as from a usability point of view, most current approaches require detailed knowledge of the integrated applications and make the implementation of a common interaction that involves different applications a difficult issue. In this paper, we propose a framework using ontologies for application integration on the user interface level by encapsulating the applications in plugins. Our approach is to use different ontologies for characterizing applications and the interactions possible with them, and for semantically annotating information objects exchanged between app...