Numerous studies have been conducted on the effect of gasoline composition on automobile exhaust hydrocarbons, in order to improve the air quality and reduce the impact on the environment. However, the quantitative relationship between gasoline composition and exhaust hydrocarbon has not been clarified. The purpose of this study is to evaluate the characteristics of individual hydrocarbons in gasoline and to clarify the effect of the gasoline composition on engine-out exhaust hydrocarbons. Experiments were performed on a single cylinder research engine operating under steady state condition. The test fuels were blended gasolines of alkylate, catalytic reformate and fluid catalytic cracking gasoline. Chemically defined binary fuel mixtures of isooctane, benzene, toluene, xylene and ethylbenzene were used as variables to study their impact on exhaust hydrocarbons. The individual exhaust hydrocarbon species were analyzed using a gas chromatograph with flame ionization detector (FID). The results of tests with blended gasoline indicated that the exhaust hydrocarbons were classified into the unburned fuel and the cracked products such as methane, ethane and various olefins. The unburned fuel of over C4 paraffins and aromatics in the exhaust was proportional to each fuel component. The regression analysis between fuel and exhaust hydrocarbons yielded the following results. The percentage of individual hydrocarbon in THC was expressed as a function of paraffin, olefin and aromatic contents in fuel. The “production coefficient” defined as the proportional constant was obtained. The production coefficient is one of the measures quantitatively relating the individual exhaust hydrocarbon to the gasoline composition. The production coefficients of benzene were 5% for toluene, 4% for xylene and 8% for ethylbenzene. These values suggested that alkylbenzene in the fuel produced benzene in the exhaust.