The effect on exhaust emissions of additives: in engine oil, in gasoline, and injected directly into the exhaust gas was studied. Substantial reduction of hydrocarbon and carbon monoxide emissions was obtained only when peroxides were injected in the presence of secondary air into the exhaust gas at the exhaust port. Fuel additives were evaluated for favorably modifying combustion kinetics, and for reducing hydrocarbon emissions related to combustion chamber deposits and quench volume. No substantial effects were found. Engine oil additive type and concentration also had no substantial effect on hydrocarbon emissions caused by combustion chamber deposits.