The variety of the European refinery processes and the different strategies adopted by oil companies to meet gasoline octane ratings have given rise to a wide range of unleaded motor fuel characteristics. This study aims to examine the relations between gasoline quality, engine base emissions and catalyst performance.Twelve fuels containing various amounts of aliphatic, olefinic, aromatic and oxygenated compounds were used in engine bench tests with a three-way catalyst. Rather large variations in the engine CO, HC and NOx base emissions were observed with the nature of the fuel used. Conversion of these pollutants by the catalyst was then determined as a function of both temperature at stoichiometry and the A/F equivalence ratio at constant temperature. It was observed that nature of the fuel influences the catalyst light-off temperature and conversions at higher temperatures.Data analysis shows that saturated hydrocarbons tend to minimize CO and NOx emissions before the catalyst, whereas olefinics give the highest values. The reverse was observed for HC. A high aromatics content increases NOx and HC emissions but is without any great influence on CO. Concerning catalyst activity, it was observed that the light-off temperature is decreased by the use of gasolines with a high olefin content, but is increased by aromatics. At higher temperatures the gasoline composition has very little or no effect on pollutant conversion at stoichiometry. Differences appear only for CO and HC on the rich side where saturated hydrocarbons give lower conversion rates.