During the past few years, web services composition has been one of the ongoing research activities in the software engineering area. It is simply defined as finding a composition of available web services that provides a requested behavior. In this paper, we study this problem for stateless web services; i.e. web services with a two-step workflow: receiving some inputs and returning some outputs as the result. Using a graph model we represent the behavior of available web services in terms of their input-output information, as well as semantic information about the web data. We also introduce a process algebra to specify the behavior of composite web services based on the behavior of simpler ones. Using the graph and the process algebra, we explain how to find useful web services for a request and how to compose them to obtain the expected behavior. We discuss the complexity of this approach and show that although it is a naturally complex process, by applying some simplifications, a reasonable overall complexity can be achieved.