Twenty-nine in-use automobiles with three-way catalyst emission control systems were misfueled with leaded gasoline in order to quantify the emissions effects. The vehicles used between four and twelve tanks of leaded gasoline. The leaded gasoline had an average of 1.0 grams Pb per gallon. Four different test programs were conducted with different misfueling intensities (rates) and mileage accumulation schedules. The Federal Test Procedure and several short tests were conducted at various stages. The results of the program indicate that vehicle emissions are mainly affected by the amount of lead passing through the engine and secondarily by the rate of misfueling.