As Web service transactions have been firmly established and widely adopted, it is important to adopt a suitable formal method to specify transaction processing in Web services. However, current research has focused on the modeling for transaction and validating of standards, the study of Web service transactional properties is still lacking. There exists a formal method of transaction processing which appends some operators and makes the syntax and reduction rules so complex that it is hard to analyze further the transactional properties. Aimed at this problem, without introducing any action operator, an extended π-calculus is proposed based on the connection between the process interactives and the transmission process of message relative to the transaction scope. And the membrane activities are defined. This biological membrane structure naturally characterizes the exception handling and compensation transactions in the multi scopes for Web services. The syntax, structural congruence and reduction rules of the extended π-calculus are given successively. According to the transactional dependency, a new weak transactional open bisimulation relationship is also presented to characterize the equivalent relationship in visible transactional action. Based on the bisimulation, its equivalency analysis is explored. All of the results can help to lay a substantial foundation for a further analysis of transactional properties for Web services.