From the interfacial tension of organic compounds with water and their contactable surface areas, the aqueous solubility of such compounds can be quantitatively predicted. Vice-versa. when the interfacial tension with water is known for a given organic compound, as well as its aqueous solubility, its contactable surface area can be obtained. From any two of these three data, and the compound's surface tension, its free energy of hydration can be calculated. However the free energy of hydration is not simply linked to a compound's aqueous solubility. For compounds which are partly apolar and partly polar, the interfacial tensions with water must be separately determined for each part; once these are expressed in kT units, they can be added together, following which the aqueous solubility of the complete compound can be calculated (a procedure which has been used earlier to obtain the critical micelle concentrations of nonionic1and anionic2surfactants). From a comparison between the solubilities (and the interfacial tensions) of alcohols and those of the corresponding hydrocarbons, it is possible to obtain the (repulsive) free energy of OH-groups interacting in water, as well as the free energy of hydration of the OH-group of alcohols. The latter is about -5·6kT, or -14kJ/Mole.