Nowadays various mobile platforms from PDA, smart phones to iPhones can deliver road warriors or trippers much mobile information. However, most mobile applications are lacking the "social intelligence" of real companionship. One reason is that community context and spatiotemporal context information are not well taken into consideration. In this paper we propose an ontology-based context model using OWL/RDF. With the enhanced interoperability substantial context information can be expressed and reasoned across systems. Based on the context model and the middleware Context-Aware Adaptation Service (CAAS), we introduce a mobile tourist guide as proof- of-concept that provides users context-aware information. We employ the MPEG-7 and MPEG-21 metadata standards to realize multimedia adaptation to device preferences. The evaluation result proves the feasibility of the context model and shows the easy extensibility of CAAS.