An important challenge for service-based systems is to be able to select services based on feedback from service consumers and, therefore, to be able to distinguish between good and bad services. However, ratings are normally provided to a service as a whole, without taking into consideration that services are normally formed by a composition of other services. In this paper we propose an approach to support the decomposition of ratings provided to a service composition into ratings to the participating services in a composition. The approach takes into consideration the rating provided for a service composition as a whole, past trust values of the services participating in the composition, and expected and observed QoS aspects of the services. A prototype tool has been implemented to illustrate and evaluate the work. Results of some experimental evaluation of the approach are also reported in the paper.