Data formats are important components for parallel event tracing tools used for debugging and performance analysis. This paper presents a generic extension mechanism for the Open Trace Format (OTF) that allows to add arbitrary data to the existing event data types. This avoids the need to frequently adapt the data format and the library API to accommodate new usage scenarios. The replay of MPI communication is demonstrated as a real-world example that requires an extension. Several other application scenarios that would benefit from additional data about certain event types are mentioned. (C) 2010 Published by Elsevier Ltd.