Semantic Web technologies promise great benefits for Personal Knowledge Management (PKM) and Knowledge Management (KM) in general when data needs to be exchanged or integrated. However, the Semantic Web also introduces new issues rooted in its distributed nature as multiple ontologies exist to encode data in the Personal Information Management (PIM) domain. This poses problems for applications processing this data as they would need to support all current and future PIM ontologies. In this paper, we introduce an approach that decouples applications from the data representation by providing a mapping service which translates Semantic Web data between different vocabularies. Our approach consists of the RDF Data Transformation Language (RDTL) to define mappings between different but related ontologies and the prototype implementation RDFTransformer to apply mappings. This allows the definition of mappings that are more complex than simple one-to-one matches.