With Web mashups, data from different Web documents and services are “mashed ” together to create a new functionality. The mashup developer usually has a clear vision of the desired output, i.e., the resulting Web page to present to the end user. Complex mashups require multiple processing steps, and become hard to debug if the delivered result is not as expected. In this paper we propose an approach that supports step-by-step debugging for declarative development of data mashups. A dependency graph is constructed from the mashup definition, and developers are able to define breakpoints to inspect a snapshot of the running mashup execution. A Web 2.0 application visualizes the intermediate results in each processing step. On top of that, it is possible to specify expected and unexpected result elements. If the result does not comply with the specifications, the platform helps to identify the processing step which caused the error.