The secondary photochemical products from alcohol fueled engine exhaust gases were investigated. Sampled exhaust gases from an S. I. engine fueled by methanol or ethanol were introduced into a reactor tube with dilution air at dilution ratios ranging between 10 and 2000. Near ultraviolet rays were irradiated throughout the samples. At high dilution ratios ranging from 1000 to 2000, the resulting photochemical product from both unburned methanol and ethanol was methylnitrate. 3-5% of unburned alcohol was converted to methylnitrate in two or three days. In the case of ethanol emission, methylnitrate seemed to be formed by photochemical reaction passes except for that of ethylnitrate as an intermediate product from ethanol. Intermittent irradiation of ultraviolet rays and coexistence of hydrocarbons in an alcohol engine exhaust had no significant influence on the photochemical reaction mechanism of unburned alcohols.