The prediction of the solubilities of pharmaceutical compounds in solvent and co-solvent systems can be achieved using a one-parameter Wilson activity coefficient model. The pharmaceutical compounds tested included salicylic acid, acetaminophen. benzocaine, acetanilide and phenacetin. The solvent employed was water, and the co-solvents studied were ethanol, methanol, and 1,4-dioxane. The solubilities of solutes in a binary mixture of water and co-solvent have been predicted on the basis of the solubility data of the solutes in pure solvent. The method was modified in order to obtain improved results using two ternary solubility data. (C) 2009 Elsevier B.V. All rights reserved.