The effect of the gasoline antiknock additive, MMT, on automotive emission control systems was studied in a 63-car field test. The cars were operated for 50 000 miles, and the effects of MMT on hydrocarbon, CO and NOx emissions, catalyst plugging and spark plug life were determined.Two concentration levels of MMT in a clear base fuel were studied, 1/32 g Mn/gal and 1/16 g Mn/gal. Seven 1977-78 model year cars, all calibrated to meet California standards, were included in the statistical design.The results of this study indicate that the use of MMT at either test concentration increases both engine and tailpipe hydrocarbon emissions, compared to clear fuel. At 50K miles, the average tailpipe hydrocarbon increase was 0.09 g/mile for 1/32 MMT fuel, and 0.11 g/mile for 1/16 MMT fuel. This increase was pronounced at low mileage intervals, and significant differences continued for the duration of the test. CO and NOx emissions, catalyst plugging, and spark plug life were not affected by MMT.