Web services (WSs) are becoming increasingly popular because of their potential in several application domains including e-Enterprise, e-Business, e-Government, and e-Science. Based on open XML standards, WS technology allows the construction of massively distributed and loosely coupled applications. A service composition mechanism should satisfy not only functional properties but also non-functional Quality of Service (QoS) ones. In this paper, we introduce a discrete-events modeling approach for Service-Based Applications (SBAs). This approach is oriented towards QoS evaluation through simulation.