Web services (WS) provide a technology for integrating applications over the Internet. This paper proposes a multi-agent conceptual model of WS and an agent-oriented modelling and formal specification method to address the difficulties in developing WS applications. The paper presents a graphic model of the general architecture of WS in agent-oriented modelling language CAMLE and an abstract specification in the formal specification language SLABS. It also illustrates how agent-oriented modelling and formal specification can be applied to the development of WS applications by an example of online auction WS and its requester application. It is shown that models and formal specifications enable software engineers to specify not only the service provider?s functionality and behaviour, but also the requirements and restrictions on service requesters? behaviour. Such semantic information is crucial for the success of dynamic integration of WS.