To clarify the thermal deterioration mechanism of three-way catalysts quantitatively, we investigated the relationship between the catalytic performance and the catalyst characteristics for thermally aged Pt/Rh catalysts. 1. Experimentally, the HC oxidation reaction, which occurs on the surface of Pt/Rh particles on catalysts, is approximated by a first-order reaction of HC concentration with a constant activation energy. 2. The relationship between the Pt mean diameter D and the HC 50% conversion temperature T50 is described by the following equation; where E and kB are the activation energy of the reaction and the Boltzman constant, respectively. 3. The sintering rate of Pt particles on the three-way catalysts was found to depend on the aging temperature(T) of catalyst and the concentration of oxygen in the gas phase([O2]). The relationship among Pt particle mean diameter(D), T, [O2] and time(t) is expressed by the following equation. where A and x are constants for the given reaction. 4. The relationship between the Pt particle mean diameter D and the oxygen storage capacity(OSC) of typical three-way catalyst is expressed as; where α is the constant. It may be stressed that the deactivation of HC conversion and the decrease in OSC of the Pt/Rh three-way catalyst strongly depend on the sintering of the Pt particles.