The service oriented model is the emerging approach adopted by Web and Grid services where parties act as service requestors and service providers. An aspect to consider is improving machine-to-machine interaction by enabling an automatic evaluation and selection of available services with respect to a set of expectations. This paper presents a model for the rigorous representation of service characteristics, for the association of each of their possible values with the requester satisfaction, and for the aggregation of the single satisfactions in an overall score using a particular logic. The proposed model is exemplified in the area of Grid computing.