—Context-aware adaptive systems aim at automatically personalizing the user’s environment depending on the user’s situation, and hence, minimizing user interaction with the system. We present a novel user profile ontology that is dedicated to describe situation-dependent sub-profiles. This ontology can be used by context-aware adaptive service platforms for mobile communication and information services to automatically trigger the situation-dependent personalization of services. The design of this novel ontology also takes into consideration recommendations from the human factors research area. In particular, the ontology enables the easy specification of situational conditions and situation-dependent user sub-profiles.