In this paper, we propose an object oriented model for describing a mobile user's context inside a Web-based groupware system. These systems are confronted to the growing use of mobile devices, such as laptops, PDAs or cellular phones, which requires special context-aware capabilities. Our model formalizes the context notion, decoupling context and system information, which consequently reduces the system development complexity. We consider the context as a set of basic elements that identify the circumstances in which the user is accessing the system. These elements include elements related to physical aspects (the user's location and device), as well as those related to the cooperative process in which the user is involved (group, role, activities, etc.).