Formaldenhyde was experimentally confirmed to be formed in a diluted exhaust gas from a methanol-fueled spark ignition engine by ultraviolet irradiation. For exhaust gases released into the air from an ethanol-fueled engine, formation of acetaldehyde took place due to photochemical reaction. The amount of aldehydes formation cannot be negligible as well as that of alkylnitrate methylnitrate or ethylnitrate. The important secondary product from an ethanol engine emission is CL-nitrate, under the conditions of ultraviolet exposure and high dilution with air.