The effects of lead and sulfur in gasoline on the activity of two platinum oxidation catalysts have been studied using engine dynamometer units. Under the steady-state conditions used, no poisoning due to sulfur was observed. Prolonged operations with lead up to 0.07 g/gal reduced the hydrocarbon (HC) conversion activity of the catalysts in proportion to time and lead concentration, but did not affect carbon monoxide (CO) conversions. The overall extent of lead poisoning was relatively minor, however. Catalysts exposed to the equivalent of 25,000 miles' operation with a fuel containing 0.07 g of lead/gal still met the original 1975 federal emissions standards of 3.4 g/mile of CO and 0.41 g/mile of HC when tested on an experimental vehicle. Exposure of platinum catalysts to exhaust from 20 gal of fuel containing 0.5 g of lead/gal caused an immediate drop in catalyst activity, but this loss was rapidly recovered when operations continued with a lead-free fuel. Thus, short-term contamination of catalyst-equipped cars with lead may not be the serious problem it was formerly thought to be.Several 1973 model vehicles have been converted by the addition of experimental oxidation catalyst systems and used to determine the effects of fuel gravity and volatility on emissions and performance. To the extent tested, these catalyst-equipped cars gave driveability and fuel economy at least as good as that demonstrated by the corresponding 1973 vehicles. At an ambient temperature of 70°F, using the 1975 federal test procedure (FTP), fuel volatility and gravity did not significantly affect HC and CO emissions for these systems.