The deactivation of noble metal oxidation catalysts by lead and halide lead scavengers was studied in engine and laboratory experiments. The halide scavengers caused rapid but completely reversible inhibition of the catalyst activity, which existed only as long as the halide was present. The effects of catalyst temperature and noble metal concentration indicated that the halide scavenger dissociated upon adsorption on the catalyst. Palladium and platinum-palladium catalysts were more susceptible to halide inhibition than were platinum catalysts. Lead alone or lead plus scavengers produced a persistent poisoning of the catalyst. Lead poisoning effects were increased by increased catalyst temperatures and fuel lead content.Tests with scavengers only, conducted in an engine previously operated on leaded fuel, showed that lead was transported to the catalyst causing lead poisoning even in the absence of lead in the fuel. These experiments showed that the reversible scavenger inhibition effects could be superimposed on the persistent lead poisoning effects.