The effect of fuel hydrocarbon composition on exhaust emissions was determined according to the Japanese statutory cycles for five Japanese vehicles using seven kinds of unleaded fuels. Vehicles used for the tests were conventional internal combustion engine vehicles with - and without - oxidation catalyst and EGR (exhaust gas recirculation), rotary engine vehicles with - and without thermal reactor and a CVCC engine vehicle. Hydrocarbon composition of the seven fuels varied from 10 to 50 vol.% in aromatics and from 0 to 25 vol.% in olefin content.The effect of fuel composition on mass emissions of CO, HC and NOx is of little significance for any of the vehicles tested. The hydrocarbon composition in exhaust gas varies substantially with the fuel composition and emission control system of the vehicles. As the fuel aromatics increase, aromatic content in the exhaust hydrocarbon fraction also increases, but olefin content tends to decrease in all vehicles except the catalyst vehicle. The specific reactivity of the exhaust hydrocarbon fraction does not vary much for any vehicle except the catalyst one, even if the fuel hydrocarbon composition varies. The catalyst vehicle shows lower aromatic and olefin contents in exhaust and consequently lower reactivity especially on the 10 mode hot cycle than other vehicles.A good relationship between olefin and aldehyde contents in exhaust organics is observed in all data obtained from this program.