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Although web services have been espoused due to their many benefits, it is known that overhead delay, particularly the communication delay, associated with invocation and execution of web services is high. Consequently, much research has been expended on minimizing those delays. In many situations an application invokes a web service repeatedly such that some or most of the data returned by the web service does not change. For instance, many web services that return schedules, such as bus or train schedules, exhibit this property. We present Differential Caches, with the accompanying Differential Updates method and the Mobile SOAP (MoSOAP) protocol, to avoid transfer of repeated data, sent by a web service to an application. The protocol is flexible in that other optimization techniques, such as encoding, can also be applied. We report on results of experiments, using a research prototype, to evaluate the method's potential benefits and also its overhead. The results of experiments show clearly that potential benefits outweigh the overhead. Under optimal conditions, the MoSOAP protocol with Differential Caches obtained a speedup of close to 800%, in delivery of the web services' replies in comparison to the SOAP communication. Further improvements in delays were gained when encoding was used in conjunction with Differential Caches.