A study has been conducted Co investigate the effect of phosphorus derived from zinc dialkyldithiophosphate (ZDP) on monolithic, three-component control catalyst durability. The objectives were to identify the extent of phosphorus induced deactivation, identify the form (s) of the phosphorus species responsible for the poisoning and study reactivation of the poisoned catalysts by thermal treatments. Catalysts were aged on an engine dynamometer for 200 hours. The experiments included accelerating oil usage by removing either the intake or exhaust valve seals and doping the oil with excess ZDP. Dynamometer evaluations show that catalysts exposed to oil (accelerated leakage through the intake or exhaust valves) with a phosphorus content of 0.7 wt. % have considerably poorer activity than a base line catalyst with very little oil exposure. Laboratory evaluations indicate that 0.4 wt. % phosphorus on the catalyst is enough to cause a significant loss in activity while additional phosphorus has only a slight effect. Attempts to reactivate the phosphorus poisoned catalysts under lean and rich synthetic exhaust conditions at temperatures as high as 760°C did not succeed.No crystalline zinc phosphorus compounds were detected by X-ray diffraction. SEM analyses of the engine aged catalysts show the presence of amorphous zinc phosphorus compounds largely on the surface of the support. The zinc phosphorus compounds found on the catalyst engine aged with intake valve seals removed are different from those found on the catalyst engine aged with exhaust valve seals removed. It also appears that species other than zinc pyrophosphates are present and are at least partially responsible for catalyst deactivation. Finally, pore plugging is observed and accounts for some of the deactivation.