An evaluation was made of the effect produced by the addition of oxygenates such as ethanol, ETBE and MTBE and nonoxygenates such as isooctane and toluene on the Reid vapor pressure (RVP) and octane number of two types of gasoline with different chemical compositions. Locally produced gasoline was blended with five different percentages (v/v) of the additives, i.e. 5, 10, 15, 20 and 25%. Ethanol and MTBE increased significantly the RVP of the mixtures, but ETBE, and particularly toluene and isooctane, decreased the RVP of the original fractions when mixed with gasoline. The octane rating of gasoline was found to increase continuously and linearly with the addition of the oxygenated compounds toluene and isooctane. Moreover, the mixture octane number of the oxygenated compounds was superior to the octane number of the same compounds of high purity grade. (c) 2005 Elsevier Ltd. All rights reserved.