The huge demand for ad-hoc and situational enterprise-class applications led to a new kind of Web-based applications, known as enterprise mashups. End users from the business units with no programming skills are empowered to combine and reuse existing company internal and external resources within minutes to new value added applications. In order to handle the growing number of mashable components, we propose a context-aware concept for enterprise mashups that supports users to find relevant components according to their current situation and to compose them automatically. The designed context model which is structured in the three domains agent, computing and environment is implemented in the SAP Research RoofTop Marketplace prototype to demonstrate its applicability and business benefits.