Content-based publish/subscribe provides a flexible communication model for component interoperation in large-scale environments. In process support systems and other applications that follow an event-based architectural style, the definition of dependencies between interacting components and the notion of all-or-nothing semantics are often needed to ensure reliable inter-component task execution. In this paper we introduce publish/ subscribe (P/S) transactions as an abstraction to support these needs in content-based publish/subscribe middleware. A P/S transaction demarcates within an atomic unit-of-work, the production, delivery, and processing of a number of related asynchronous event notifications. A transaction service, provided by the middleware, realises P/S transactions to support the transactional execution of processes on behalf of applications.