Web services represent an alternative basis for the rapid development of application systems. Much of the research on composition and orchestration of web services centers around functional sufficiency and performance. Non-functional characteristics like reliability and security play an important role in the selection of web services by system architects. This paper provides a basis for measuring reliability of an application system that is assembled using web services. It outlines criteria for measuring reliability for individual tasks, specifies combining functions for differently configured tasks, and applies these techniques to a problem which contains various sizes of business processes to demonstrate the efficacy of the approach.