This paper presents an alternate method for estimating RON using the reformate density, as an indirect measure of its composition. Reformate samples were obtained from catalytic reforming of naphtha in a pilot plant using different catalysts at various temperatures. Actual RON by engine test of these reformates and their densities were determined. A power law type of equation gave the best correlation (correlation coefficient = 0.94, standard deviation of residuals = 0.45) with the octane numbers determined by ASTM engine tests. The benefits and limitations of the method, in comparison to other methods, are also discussed. It was concluded that, while the method based on density gives comparable accuracy, it is simple, rapid and economical.