By comparing model and real converter performance, concise models of three-way catalyst (TWC) dynamics permit a more reliable diagnosis of converter aging than conventional approaches. Also, future model-based engine control systems should manage the state of the TWC in a way to reduce emissions. For model validation, results of transient gas concentration measurements on a dynamic test bench with an SI engine are shown. To identify the occurring fast transient phenomena, very fast multichannel gas analyzers must be used. Simulation results using a recent model of the catalytic converter are compared with actual measurements and lambda sensor readouts.