A review of difficulties experienced with proprietary guidelines languages and in clinical decision support has led us to the development of a decision support system (DSS) based on a service oriented architecture (SOA) using Business Process Execution Language (BPEL) and Web services. BPEL allows simple branching conditions and tests to be set based on the information artifacts in the workflow. For more complex tests, we preferred to place decision support in an external DSS Web service. An advantage of this approach is that the DSS could evolve independently of the clinical workflow. We have implemented an exemplar of the above using the Java expert system shell (JESS). The domain objects are sent by the BPEL engine into the DSS which consults external rules, reasons on the domain objects and rules, and transmits recommendations back. The article will discuss issues involved in implementation using as an example drug-drug adverse reactions.