The regular solution model (the Hildebrand-Scatchard equation) has been improved so that it can be applied to polar mixtures by introducing the exponent-type mixing rule and interaction parameters between unlike molecules. The applicability of the proposed regular solution model is examined by correlating vapor-liquid equilibria of several ethanol + hydrocarbon binary systems, and it is found that the proposed model can be adopted for polar mixtures. For calculations, solubility parameters and liquid molar volumes at given temperatures can be estimated from the molecular structures of constituent pure components by using additive methods.