Behavioural protocols are beneficial to Component-Based Software Engineering and Service-Oriented Computing as they foster automatic procedures for discovery, composition, composition correctness checking and adaptation. However, resulting composition models (e.g., orchestrations or adaptors) often contain redundant or useless parts yielding the state explosion problem. Mechanisms to reduce the state space of behavioural composition models are therefore required. While reduction techniques are numerous, e.g., in the process algebraic framework, none is suited to compositions where provided/required services correspond to transactions of lower-level individual event based communications. In this article we address this issue through the definition of a dedicated model and reduction techniques. They support transactions and are therefore applicable to service architectures.