This paper deals with the experimental study that aims to examine the effects of octane number of three different fuel oxygenates on exhaust emissions of a typical spark ignition engine. Three commonly used oxygenates, namely methyl tertiary butyl ether (MTBE), methanol, and ethanol, which were blended with a base unleaded fuel in three ratios (10, 15 and 20 vol%), Were investigated. The engine emissions of CO, HC, and NOx were measured under a variety of engine operating conditions using an engine dynamometer set-up. It is found that generally as the octane number of the fuel increases the CO and HC emissions decrease but the NOx emission increases for all three blends. Further, for the leaded fuel (RON of 92), as the speed of the engine increases the CO and NOx emissions decrease but the HC emission decreases. A similar trend was found for MTBE blends also. These emission results are presented in terms of octane number and their effects are discussed in this paper. Copyright (C) 2002 John Wiley Sons, Ltd.