To clarify the gap between the effective oxygen storage capacity (OSC) and complete OSC of Pt/CeO2-ZrO2 (CZ) catalysts, a quick-response reactor coupled with a high-time-resolution mass spectrometer was used to measure oxygen storage and release by flushing CO and O-2 alternately and periodically. Hence, both oxygen storage and release were visualized together and the effective OSCs were determined based on the breakthrough influxes of CO and O-2. The obtained effective OSCs varied with the CO concentration and were much lower than the complete OSC as determined by the conventional CO2 integration method. In accordance with the high oxygen mobility, as determined by the O-18/O-16 isotopic exchange reaction, the Pt catalysts supported on CZ oxides with a homogeneous and ordered Ce-Zr structure exhibited very high OSCs even at very low Pt loadings and low CO concentrations.