Experiments were carried out to evaluate the effects of 2-ethylhexyl nitrate and di-tertiary-butyl peroxide on the exhaust emissions from a vehicle equipped with a common rail diesel engine.A base fuel in compliance with European 2000 specifications was additized with the two cetane improvers. Additive concentrations were calculated to increase the cetane number by 6.1 points.Emissions were measured with and without the catalytic converter using the New European Driving Cycle (NEDC) test procedure.Both cetane improver additives have the same effect on emissions. Carbon monoxide (CO) and unburned hydrocarbons (HC) are reduced. Particulate matter (PM) after catalysts is not improved and nitrogen oxides (NOX) emissions increase slightly. Consumption remains unchanged.The cetane improver additives effect on HC and CO is maximum during the ECE cycle, when catalyst efficiency is low.