Abstract: Oxidation catalysts containing platinum or other noble metal components used in automobile exhaust emission control are reactivated, after thermal deactivation, by subjecting the catalyst to a halogen-containing gas at a temperature of from 500.degree.F. to about 1000.degree.F. The passing of carbonyl halide or a mixture of carbon monoxide and halogen, alone or in a carrier gas, over the catalyst reduces the CO-oxidation and hydrocarbon-oxidation temperature to almost that of fresh catalyst. Carbonyl chloride or the carbon monoxide-chlorine mixture is preferred. This treatment may also be used in the reactivation of noble metal-containing reforming catalysts.