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. With regard to the fuel composition, the need to maintain an acceptable octane number requires an increase in heavy alkanes in order to reduce the aromatic content. Since alkanes decompose and form alkenes with high MIR values, reducing the aromatic content is not so effective in reducing the specific reactivity. However, it does affect the reduction of ozone formation in the air as a result of reducing NMOG. The NMOG level is higher with M85 than with gasoline. However, since the specific reactivity is low, using M85 results in less ozone formation than with gasoline. The specific reactivity of M85 vehicles is influenced mostly by formaldehyde emissions.