During recent years, the World Wide Web (WWW, Web) has increasingly been used as a platform for applications that link together processes both between and across organizations. Acting as distributed components, Web Services provide a standardized way of externalizing functionality on a global scale and as such enable accesses that transcend organizational boundaries to form federated applications. The design and evolution of these federated applications is now imposing new obligations for the disciplined engineering of composed Web solutions. To meet these obligations, we extend the WebComposition idea, which is an approach to apply component-based software development concepts on Web applications. This extension facilitates modeling the complex landscape of the components and services building the federated applications. In this context, we introduce the WebComposition Architecture Model that serves as a map to keep track of the interrelations between the federated partners in terms of the involved Web-technology. Among the modeled artifacts are Web services, Web Applications and organizational zones of control that are all subject to evolution in the sense of the WebComposition approach.