The growing diffusion of portable devices enables users to benefit from anytime and anywhere impromptu collaboration. Appropriate policy models that take into account the dynamicity and heterogeneity of the new pervasive collaboration scenario are crucial to ensure secure sharing of information. Collaborating entities cannot be predetermined and resource availability frequently varies, even unpredictably, due to user/device mobility, thus complicating resource access control. Policies cannot be defined based on entity?s identities/roles, as in traditional security solutions, or be specified a priori to face any operative run-time condition, and require continuous adjustments to adapt to the current situation. To address these issues this paper advocates the adoption of a semantic context-aware paradigm to policy specification. Context-awareness allows operations on resources to be controlled based on context visibility whereas semantic technologies allow the high-level description and reasoning about context/policies. The paper describes Proteus that, as a key feature, combines these two design guidelines to enable dynamic adaptation of policies depending on context changes. In particular, the paper shows how ontologies and logic programming rules can be used to leverage policy adaptation.