Abstract. For the adaptation of services to the current situation of a user, the services are in need of specific context information. The acquisition of context in highly dynamic environments is a complex process as the appropriate context sources are not known in advance. Moreover, to realize Mark Weiser's vision of ubiquitous computing, many services on the one hand and a good deal of context information on the other hand have to be combined. Hence, we follow a middleware approach to automate context retrieval for services. For the exchange over domain boundaries, services in need of and services offering context information have to agree on a common description of the information. Therefore, a flexible and extensible information model is a basic requirement. This paper describes in detail the integration of those two important foundations of context-aware computing.