Reid vapor-pressure (RVP) measurements of simulated;gasoline with a variety of additives and fuel extenders, including alcohols, ethers, ketones,amides, amines, and esters, at different concentrations and 100 degrees F using a Setavap Vapor Pressure Tester 22420-3 are presented. Prediction of RVP of simulated gasoline in the presence of the different additives and fuel extenders has been performed using three UNIFAC-based models. Satisfactory predictions are obtained when nonassociating or weak associating additives are involved (average absolute errors in the prediction of RVP of the order of 1-2%), with the most satisfactory being the model of Gmehling et al. (Ind. Eng. Chem. Res. 1993,32,178-193). The results are. poorer, however, for mixtures containing strongly associating additives such as methanol, ethanol, 2-propanol and 2-methyl-1-propanol tin many cases the absolute errors in the prediction of RVP were over 5%). UNIFAC models can be used for initial screening of potential additives, but actual effects on gasoline vapor pressure should be determined experimentally.