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Web services have become an integral part of workflow orchestration in scientific applications and many tools used by scientists including Kepler, Taverna and Triana incorporate Web services. However orchestration of complex workflows that involve services that exploit standards like WS-Addressing or WS-Security are not easily managed by these tools. Most complex use cases that involve ldquoadd onrdquo service standards use implementation specific assumptions, and as a result, the services and workflow composers become tightly coupled. This leads to stove-piped, noninteroperable implementations. This paper describes an effort to implement complex use cases that include asynchronous messaging and WS-Security, for a large application project called LEAD, while maintaining standard conformance and composer simplicity. The primary contribution of the paper is design of a Mediator and a generic Web service actor that allow the addition of new Web service standards to services in a workflow, without the need to make the workflow composer or enactor explicitly aware that these standards are being used. These concepts are demonstrated by an implementation that allows large workflows to be constructed using two different scientific workflow systems that were not designed with these extensions in mind.