This document describes a new architecture that exploits Semantic Web technologies for supporting pervasive context-aware systems. This architecture called Context Broker Architecture (CobrA) differs from other architectures in using the Web Ontology Language OWL for modeling ontologies of context and for supporting context reasoning. Central to our architecture is a broker agent that maintains a shared model of context for all computing entities in the space and enforces the privacy policies defined by the users when sharing their contextual information. We describe the use of CoBrA, its associated ontologies, and its privacy protection mechanism in an intelligent meeting room prototype.