Results of three-way catalyst (TWC) conversion efficiency measurements are presented which demonstrate the effects of dynamic operation. In one series of measurements, the time-averaged conversion efficiencies were determined for the case in which the TWC was subjected to a series of complex air-fuel ratio (A/F) waveforms. The measured efficiencies are compared to predicted efficiencies determined from the superposition of the TWC response to the individual sinusoidal components comprising the complex waveform. This comparison shows discrepancies for CO in the lean region, and for HC and NOx in the rich region.In a second series of measurements, the instantaneous conversion efficiencies were determined for a square wave A/F waveform using a timed-sampling technique. The results of these measurements illustrate the effects of O2 storage, the enhancement of CO conversion due to the water-gas shift reaction, and the enhancement of NOx conversion during the rich portion of the A/F waveform. The square wave A/F results are used to partially explain the time-averaged results.