The deactivation of a three-way catalyst (TWC) included in warm-up catalytic converters (WCC) employed in passenger vehicles has been examined with respect to the catalyst field mileage by drivers. By a sweep test (ST) under the A/F oscillation of 1 Hz simulating the actual operation of a gasoline engine, the gradual deactivation of CO oxidation activity under rich condition over the Pd TWCs customer-aged was clearly observed with respect to the catalyst mileage, while the oxidation activity of C3H6 had been hardly altered. No proportional dependence of the deactivation of the CO and C3H6 oxidation activity on the catalyst mileage, however, was determined by both a steady-state sweep test (st-ST) and a light-off test (LOT) without A/F perturbation, although the initial deactivation of both reactions by both test modes was apparent. The sintering of noble metal (NM) itself might not be the only cause for the deactivation of TWC with respect to the catalyst aging mileage. The gradual alteration of the oxygen storage capacity (OSC) of TWC. mainly due to the degradation of the oxygen storage components and the weakness of the Pd-Ce interaction, is also the cause for TWC deactivation, particularly for the CO oxidation reaction under rich condition. No deactivation of the oxidation reaction of C3H6 by ST, regardless of the catalyst mileages is likely due to the strong oxidative capability of I'd toward hydrocarbons. (C) 2009 Elsevier B.V. All rights reserved.