Web services are nowadays one of the preferred technologies to implement service-oriented architectures and to communicate distributed applications. On the other hand, context-awareness is highly demanded for distributed applications. However, even though there are excellent tools and frameworks for service development, getting services to be context-aware is still under investigation. In turn, an Enterprise Service Bus (ESB) is a standards-based integration platform, which provides mediation capabilities (e.g. routing, transformation). ESBs are being increasingly used in conjunction with Complex Event Processing (CEP) engines to support event-driven architectures scenarios. In this regard, this paper proposes an ESB-based integration platform which, leveraging its mediation capabilities and a CEP engine, allows the construction of context-aware web services. Concretely, CEP techniques are used to detect the complex situations that may affect services and mediation mechanisms are used to adapt service requests and responses to make them context-aware.