The equilibrium solubility of the pharmaceutical substance, (S)-(+)-6-Methoxy-alpha-methyl-2naphthaleneacetic acid (naproxen) in solvent consisting of acetone and water at different volume ratios is determined under different temperatures from 0 C to 20 C in increments of 5 C. NRTL and Wilson models can describe the equilibrium of vapor-liquid and liquid-liquid systems successfully. In this paper they are used to correlate solid-liquid equilibrium for naproxen in acetone and water mixtures. The results show that NRTL and Wilson models correlate the solubilities of naproxen in acetone and water mixtures successfully under different temperatures. The interaction parameters between naproxen and acetone for Wilson and NRTL models are obtained. (C) 2014 Elsevier B.V. All rights reserved.