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The gas velocity through an automotive catalyst has been determined by measuring the time of flight of a pulse of propane injected at the inlet plane of the catalyst. The arrival time at the exit plane was detected by a fast flame ionization detector. By synchronizing and delaying the injection of propane with respect to the engine crankshaft position, the fluctuations of the exhaust gas velocity during the engine cycle were investigated. A number of tests at different engine load and speed points were carried out. The results show a complex velocity/time characteristic, including flow reversals. The technique is shown to be a viable option for flow measurement in this harsh environment.