The study of the effects of lead on noble metal automotive oxidation catalysts has been continued using engine and chassis dynamometer units as well as laboratory evaluations. Work has been carried out with monolithic and pelletized catalysts and with lead levels ranging from trace to 3.5 g/gal.Engine and chassis dynamometer studies with monoliths have confirmed earlier findings that trace lead up to the 0.07-0.10 g/gal. level has almost no effect on activity maintenance during long term aging. At higher trace levels, deactivation effects become apparent over the long term. However, brief exposures to fully leaded fuels in dynamometer or customer testing do not seem to affect catalyst life. Thus, single tankfuls of fuel at up to 3.5 g/gal. had no measurable permanent effect on catalyst activity measured by the Federal Test Procedure, although conversion efficiency was temporarily lost while the leaded fuels were in use and some permanent loss in steady state conversion was noted. Halide scavenger compounds used in conjunction with lead additives are responsible for much of this temporary activity loss.Initial testing of beaded catalysts indicates they are no more sensitive to traces of lead than the monoliths. Catalysts subjected to dynamometer aging for the equivalent of 25,000 miles with fuel containing 0.10 g/gal. of lead easily met CO and HC emission standards of 3.4 and 0.41 g/mi respectively in vehicle FTP testing.Laboratory studies of aged catalysts showed a correlation between lead content and activity, with the first increments of lead causing the greatest deactivation. These results contradict vehicle test results showing a linear deactivation trend with lead accumulation, accentuating the difference between laboratory and vehicle studies of lead effects. An effect on HC conversion activity of oxygen concentration was also found, which may be relevant to the quantity of excess air supplied to vehicle mounted catalysts.