An engine-based test has been developed to measure the oxygen storage capacity of a catalyst. The test utilizes the difference in the engine-out and tailpipe A/F ratios following rich-to-lean and lean-to-rich A/F transitions in order to quantify the storage or release of oxygen. The technique also results in the determination of the water-gas shift constant for the tailpipe exhaust. The technique was used to measure the oxygen storage capacity of a fresh catalytic converter at inlet temperatures of 400, 500, and 600°C for catalyst volumes of 1.5L and 2.8L. The procedure was repeated after the converter had been aged at an inlet temperature of 800°C for 20, 40, and 60 hours. The oxygen storage capacities are related to the emissions performance of the converter on A/F ratio sweep tests.For the fresh converter, the calculated oxygen storage capacity increased with temperature. The measured amount of fresh oxygen storage at 600°C was close to the theoretical maximum level for the catalyst washcoat tested. After aging for 20 hrs at 800°C, the oxygen storage capacity at 400°C and 500°C decreased significantly. With further aging, however, the oxygen storage capacity at these temperatures remained fairly steady. The HC, CO, and NOx conversion performance as measured on A/F ratio sweep tests remained very high at all aging conditions, implying that the residual oxygen storage capacity was sufficient for the A/F perturbations on the sweep test.The water-gas shift constants for the fresh 2.8L converter at 400, 500, and 600°C were very close to the equilibrium values at those temperatures. The water-gas shift constants increased with aging, suggesting that the aging decreased the ability of the catalyst to promote the water-gas shift reaction.