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The time-average conversion efficiencies of an automotive three-way catalyst subjected to exhaust flows for which A/F is modulated around stoichiometry are benefited by the ability of the catalyst to store oxidizing and reducing chemical species. A V-8 engine equipped with a vapor carburetor and exhaust system containing zirconia exhaust-gas-oxygen sensors before and after a three-way catalyst have been used to study the response of the catalyst to fast changes in exhaust A/F ratio. With this experimental arrangement, qualitative and quantitative observations of storage of oxidizing and reducing chemical species on Engelhard Industries TWC-19D3 catalysts have been made. The quantitative value for the oxygen storage capacity obtained for full-sized catalysts in real exhaust gas compares favorably with the value measured using a laboratory mass-spectrometer technique with small catalyst samples and synthesized exhaust gases.