A system is described for selecting web services accessible over a network (380). The system comprises a pervasive device (10), one or more servers (20) in communication with the network (380) and supporting at least one base web service (502) that provides an output in reply to a request from the pervasive device (10). The system further comprises one or more servers (20) supporting at least one extended web service (606) operable to reduce a computational load of the pervasive device (10) in an interaction with the base web service (502). An application (504) running on the pervasive device (10) evaluates attributes of a Quality of Service (QoS) model relating to interactions between pervasive device (10), a base web service (502) and corresponding expanded web service (606). The application (504) chooses whether or not to use the corresponding web service based on the evaluated attributes.