The solubility ratio of organic chemicals in an organic solvent and in water, sigma, has been found to correlate with the logarithm of the octanol-water partition coefficient (log K-ow) of the solutes; i.e., log sigma = N + M log K-ow. A total of 607 sets of thousands of solubility data were extracted from literature sources. From this database, values of N and M are obtained for 15 completely water-miscible cosolvents which are often used in pharmaceutical and environmental activities. A linear free energy relationship between sigma and log K-ow is theoretically derived. The deviation of the slope M from the theoretical value of unity seems to originate mainly from the differences in activity coefficients of solute in water-saturated octanol and in pure cosolvent. The effect of dissolved octanol in water in defining K-ow may also influence the slope of sigma similar to log K-ow regression.