<div class="htmlview paragraph">The thermal deactivation of current automotive three-way catalysts (TWCs) was studied under various high temperature conditions to determine which were most damaging. The catalysts were aged on an engine dynamometer simulating the U.S. emission durability cycle with additional periodic exposure to high temperatures. The deactivation was measured as a function of the duration, temperature and air-fuel ratio during the high temperature exposure. With lean air-fuel ratios during the high temperature exposure, TWC performance as measured at 600 F was most susceptible to deactivation showing appreciable loss of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) conversions after 20 minutes of aging at 1600 F. The TWC performance as measured at 800 F had comparable loss only after 60 minutes of aging at 2200 F. With rich air-fuel ratios the TWC performance remained nearly unaffected by aging up to 2000 F, but it dropped substantially after aging at 2200 F. The results are explained in terms of the loss of active metal surface area and the rates of the various reactions involved in TWC operation.</div>