Web service architecture offers advantages over traditional architectures in business-level interoperability across organisations. It has been commonly believed to be the one technology to implement future enterprise systems. Thus the effort to develop and standardise web services is overwhelming. More than 100 web service (WS-*) specifications have been introduced in the past few years. The aim of this work is to bridge the industrial specifications and real implementations using formal models. A formal model is important here to provide a high level abstraction of the system behaviours that could be applied to various web service specifications. It also helps to understand a system and further analyse it by converting the model into code used by traditional model checking tools. We introduce a new formal notation, Document Flow Model (DFM), to model asynchronous web service composition. Unlike other web service composition languages, our DFM not only captures the common service-oriented system behaviours, such as asynchronous communication, but also addresses the design issues of dynamic configurations, and long running business interactions in particular. In addition, we develop a formal operational semantics for the DFM specification describing the possible behaviours of a system composed of inter-related web services which helps to further analyse and simulate a system that is composed of asynchronous web services.