Service-Oriented Architectures (SOA) provide infrastructures to make resources available to other participants in the network as independent services. However, service providers, not having the autonomy to decide who they collaborate with, might be reluctant to participate in such open systems, the client being the sole responsible of the selection of services for the composition. Multiagent systems research offer some solutions in term of trust and reputation mechanisms as well as in coalition formation theory. This paper presents a multiagent based negotiation model to enable provider autonomy in composite web services. QoS-based reputation is built from both feedbacks retrieved from execution and from subjective feedbacks given by the client. This model is illustrated by an example based on the Language Grid Project, an service infrastructure for language resources.