Performance and the compatibility of methyl tertiary butyl ether (MTBE) as a blending component for motor gasolines were assessed for Japanese passenger cars. MTBE was found to be an excellent road octane booster. But fuel comsumption and exhaust emissions of MTBE-containing gasoline were observed to differ from conventional gasoline. The addition of MTBE changed the equivalence ratio of the mixture and reduced the calorific value of the fuel. Engine cleanliness, ORI and crankcase oil deterioration were not influenced by MTBE gasoline. MTBE gasoline was compatible with the elastomers and metals used for fuel system of Japanese passenger cars and was found to behave similarly to conventional gasoline. In general, MTBE was judged to be an excellent octane appreciator in motor fuel for Japanese passenger cars.