This paper presents a context-based awareness mechanism designed for users who access web-based collaborative systems using mobile devices. The limited capabilities of such devices (reduced display size, limited battery and memory capacity...) constitute intrinsic constraints which make even more tenuous the issue of delivering relevant information to collaborative mobile users. We propose here an object-based representation of the notion of context and an awareness mechanism which filters the available information according to both the physical (user's location and device) and organizational (knowledge relative to the collaborative process) contexts of the user.