Event-based systems (EBS) enable companies to respond to changes in their environment in a timely manner. To interpret event notifications, knowledge about their context is essential. The matching mechanisms of publish/subscribe systems depend on a common interpretation of event notifications and subscriptions that may span organisational boundaries. To mediate between such semantic contexts, we developed ACTrESS, a distributed middleware addon for automatic context transformation in event-based software systems and message-oriented middleware (MOM) in general. Transformations are substitutable at runtime and transparent to the user. ACTrESS is built on top of a production strength open source MOM extending the Java Message Service API. In this paper we present the challenges arising from differing contexts in event-based systems. We introduce ACTrESS and evaluate our solution using workloads derived from findings from research projects dealing with real-world applications of EBS.