In this study, the effect of using higher-octane gasoline than that of engine requirement on the performance and exhaust emissions was experimentally studied. The test engine chosen has a fuel system with carburettor because 60% of the vehicles in Turkey are equipped with the carburettor. The engine required 91-RON (Research Octane Number) gasoline was tested using 95-RON and 91-RON. Results show that using octane ratings higher than the requirement of an engine not only decreases engine performance but also increases exhaust emissions. (C) 2004 Elsevier Ltd. All rights reserved.