In order to ensure safety of critical real-time systems it is crucial to verify their temporal properties. Such a property is the Worst-Case Execution Time (WCET), which is obtained by architecture-dependent timing analysis and architecture-independent flow fact analysis. In this article we present a WCET annotation language which is able to express such information originating from the user or the analysis. The open format, named FFX to stand for Flow Facts in XML, is portable, expandable and easy to write, understand and process.We argue that FFX allows to reuse and exchange the annotation files among WCET tools. FFX therefore permits to tighten WCET results and decreases the effort to support new architectures. Additionally, FFX flow fact files allow fair comparisons of both flow facts and WCET results. FFX can be used for quality assurance when developing new analysis techniques, using it as a flow fact database to test against. We present a small case study exemplifying the above points. Our case study puts special focus on the aspect of comparability and information exchange among WCET tools. In our experiments with FFX, we use the WCET analysis tool chains Otawa/oRange and r-TuBound/CalcWCET167.