The solubilities of ibuprofen and diazepam in water/ethanol binary solutions were predicted using COSMO-RS and UNIFAC. In case of the ibuprofen system, RMSE was 0.183, and COSMO-RS reproduced the experimental values well. The RMSE of UNIFAC was 0.628, and the solubilities predicted by UNIFAC were smaller than experimental values. Polarizabilities of several drugs in vacuum, water, and ethanol were calculated. A linear correlation between polarizabilities and solubilities was observed to exist in water.