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Hydrocarbons and other organic materials emitted from S.I. engines cause ozone to form in the air. Since each species of organic materials has a different reactivity, exhaust components affect ozone formation in different ways. The effects of exhaust emission control devices and fuel properties on speciated emissions and ozone formation were examined by measuring speciated emissions with a gas chromatograph and a high-performance liquid chromatograph. In the case of gasoline fuels, catalyst systems with higher conversion rates such as close-coupled catalyst systems are effective in reducing alkenes and aromatics which show high reactivities to ozone formation. With deterioration of the catalyst, non-methane organic gas (NMOG) emission increases, but the specific reactivity of ozone formation tends to decrease because of the increase in alkane contents having low MIR values.