The mashup paradigm allows end users to build custom web applications by combining data-exchanging components in order to fulfill specific needs. Since such building blocks typically originate from different third party vendors, compatibility issues at component interface level are inevitable. This decreases re-usability and requires skilled users or automatisms to provide the necessary mediation to solve such issues. However, current mashup proposals are very limited in this regard.We present techniques for data mediation that leverage semantically annotated interface descriptions to overcome a high degree of interface mismatch. We equipped the EDYRA mashup platform for end user development with automatic support for these techniques to increase the re-usability of components and to foster the long tail of user needs. In order to show the practicability of our approach, we describe the platform implementation and present benchmark results.