The Business Process Execution Language (BPEL) uses XML to specify the data used within a process and realizes data flow via (globally) shared variables. Additionally, assign activities can be used to copy (parts of) variables to other variables using techniques like XPath or XSLT. BPEL for Semantic Web Services (BPEL4SWS) employs SAWSDL to give meaning to data by referring to ontological concepts and to enable a seamless mapping of XML data and its ontological representation. In this paper we show how this ontological knowledge can be used to ease the definition of data flow in BPEL.We therefore extend BPEL and introduce the concept of mediaton as a first class citizen. We give an example of data mediation in BPEL processes and show how process modellers can benefit from the ontological knowledge when specifying data manipulation declaratively instead of having to implement data manipulation each time a process is modelled.