A new method to measure the amount of oxygen storage in/release from catalysts on a millisecond scale per two CO pulses (MS-OSC) has been developed. The MS-OSC measurement is based on the principle of the fast detection of reaction products on catalyst surfaces just after CO and O-2 injections using pulsed valves by time-of-flight mass spectrometry. Each catalyst was slurry-coated on planar cordierite substrate. MS-OSC was measured under high-vacuum (10(-7) Pa) conditions, which allowed the MS-OSC to be directly measured as the amount of CO2 without diffusion effects. We applied the MS-OSC to the following three catalysts: Pt/CeO2ZrO2 mixture, Pt/CeO2-ZrO2 solid solution, and Pt/CeO2-ZrO2Y2O3 (Pt/CZY) solid solution. Pt/CZY was reported to improve the transient catalytic activity on a millisecond scale, which could not be explained by the result of conventional thermogravimetric OSC measurements on a second scale. From a comparison of the three catalysts, the MS-OSC of Pt/CZY was found to be the highest. This result is supported by a previously reported engine test result (H. Tanaka, I. Tan, K. Yamada, and M. Yamamoto, ASATA 98ATE018,1998).