This paper introduces a novel approach for modelling and specifying behaviors of Web services. This approach excludes Web services from any composition scenario and sheds the light on two types of behaviors: control and operational. The control behavior illustrates the business logic that underpins the functioning of a Web service, and the operational behavior regulates the execution progress of this control behavior by stating the actions to carry out and the constraints to put on this progress. To synchronize both behaviors at run-time, conversational messages are developed and permit conveying various details between these two behaviors. A prototype showing the use of these conversational messages is presented in this paper as well. © 2009 IEEE.