When Web services are composed, transactions are needed to maintain the consistency of the distributed data in all but the most trivial cases. Today?s Web service transaction speci?cations still leave some issues unresolved, e.g., the speci?cation of quality of service aspects. Therefore, we propose a modeling methodology that forces the software architect to address such issues at an early stage of development. The methodology is divided into four layers of UML diagrams which reference each other: structure, transactions, security, and work?ow. This separation of concerns can also be used to incorporate the knowledge of several experts into the design.