Knowledge about the thermodynamic properties of a polymer solution is essential to its applications. These properties are derived on the basis of a statistical mechanics approach by modifying the classic Flory-Huggins theory in this study. The analytical result derived is fitted to the available experimental data such as toluene-polystyrene and benzene-silicone polymer solutions in the literature, and its performance is satisfactory. We show that the inconsistency between the Flory-Huggins theory and experimental observations for many polymer solutions might arise from the negligence of the effects of the free volume fractions of solvent and polymer and the change in the disorientation entropy of polymer molecules. (C) 2003 Elsevier Ltd. All rights reserved.