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Negotiation is a crucial stage of Web services interaction lifecycle, aiming at establishing a formal contract between a service provider and a consumer, to specify agreed terms on the service, particularly terms on non-functional aspects. This paper introduces a formal negotiation model for Web Services. The model employs fuzzy truth propositions to specify constraints attached to the service, and uses a utility function to represent the negotiatorÂ¿s preferences over service attributes. Strategies for evaluating proposals and generating counter-proposals are also presented. In particular, the strategies for counter-proposal generation can make adjustable rates of concession, by reacting to an over-changing environment. At each negotiation round, these adaptive strategies first determine the concession at the overall utility level, considering factors of time, resource and proposals from the opponents. Then according to weight of each service attributes, the overall concession is distributed over these attributes. A set of experimental tests is conducted to evaluate the performance of the flexible negotiation strategies.