The current paper examines the effects of using MTBE as a replacement of lead additives in gasoline on exhaust emissions of a typical SI engine. The MTBE was blended with a base unleaded fuel in three ratios (10, 15, and 20 vol. percent). The emissions of CO, HC, and NOx were measured at a variety of engine operating conditions using an engine dynamometer setup. The results of the MTBE blends were compared to those of the base fuel and of a leaded fuel prepared by adding TEL to the base. With respect to the base fuel, the addition of MTBE decreased the CO emissions, decreased the HC emissions at most operating conditions, but generally increased the NOx emissions. The emissions results for the leaded fuel were comparable to those of the base fuel. [S0195-0738(00)00103-5].