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The ability to substitute a service for another is one of the features of Service-Oriented Computing (SOC). In this paper, we study the problem of degraded service substitution assuming that services have explicitly-defined interaction protocols, e.g., in the form of WS-BPEL business protocols. To this end, we characterize the behavior of interacting services by means of contracts specifying the allowed sequence of message sending and receiving operations. The contribution of this paper is a theory of contracts for service substitution which improve system survivability by gracefully degraded substitution.