The evolution of European requirements for the reduction of spark ignited engine emissions makes necessary a better understanding of the impact of gasoline formulation on regulated, photochemical and toxic pollutants.To this end, the effects of gasoline have been studied on a European four-cylinder engine, fitted with a three-way catalyst and operated at steady state conditions.A design for experiments with mixtures made it possible to study the effects of different classes of gasoline compounds, using n-pentane, isopentane, 1-pentene, cyclohexane, n-octane, isooctane, toluene, ethylbenzene, m- and p-xylenes and o-xylene introduced in an alkylate base stock.Aromatics, especially the more substituted ones, produce the greatest engine-out hydrocarbon emissions. They also increase the nitrogen oxide emissions. However, both unburned hydrocarbon and tailpipe nitrogen oxide emissions are decreased with aromatics. Aromatics are also the main precursors of exhaust benzene, due to the dealkylation of substituted aromatics. Aromatics lower the exhaust formaldehyde and acetaldehyde emissions, but aromatic aldehyde levels are increased.Increasing the olefin concentration of fuel increase engine-out nitrogen oxide and 1,3-butadiene emissions. For this last component, cyclohexane and 1-pentene are found to be the main source. Engine-out hydrocarbon emissions are reduced with C5 hydrocarbons. Exhaust olefins are decreased with aromatic fuels.Isooctane is found to be the most important precursor of isobutene, acetone and methacrolein.This work carried out in I.F.P., is included in the French Programme “VEHICULE PROPRE ET ECONOME” with ADEME, ELF, ESSO SAF, P.S.A., RENAULT S.A., Société des Pétroles SHELL and TOTAL as partners.