Recently the term Web Services choreography has been introduced to address some issues related to Web Services composition and coordination. Several proposals for describing choreography for business processes have been presented in the last years and many of these languages (e.g. BPEL4WS) make use of concepts as longrunning transactions and compensations for coping with error handling. However, the complexity of BPEL4WS makes it difficult to formally define this framework, thus limiting the formal reasoning about the designed applications. In this paper, we formally address coordination among Web Services with particular attention to Web transactions. We enhance our past work - the Event Calculus - introducing two main novelties: i) a multicast event notification mechanism, and ii) event scope names binding. The former enables an easier specification of complex coordination scenarios — such as business-to-business applications require — while the latter allows many new interes...