The explosive popularity of mashups has given rise to a plethora of ``mashup platforms''. Using these web-based tools, mashups can be rapidly constructed with minimal programming effort. Reo for Mashups (ReforM) provides tools to combine, filter and transform web services and data sources like RSS and ATOM feeds. Whereas other mashup platforms intermingle data transformation logic and I/O concerns, we aim to clearly separate them by formalising the coordination within a mashup, thereby constructing a mashup's core from a library of prebuilt connectors.