Despite important regulatory and technical efforts aimed at preserving privacy, privacy leakage incidents on the Web continue to make the headlines.we present a formal model for preserving privacy in Web services. We define a Web service- aware privacy model that deals with the privacy of input data, output data, and operation usage. We introduce a matching protocol that caters for partial and total privacy compatibility. We propose also a negotiation model to reconcile services' privacy in case of incompatibility.