We propose a new Helmholtz energy of mixing equation following the original Flory-Huggins (F-H) closed-packed lattice model. Also, to overcome F-H mean-field approximation, we introduce new universal constants to consider chain length dependence of polymer in solvent and consider specific interactions to describe strongly interacting polymer systems. Our proposed model successfully describes liquid-liquid equilibria (LLE) for binary polymer-solvent systems using identical interaction parameters which do not depend on the polymer molecular weight. We also describe vapor-liquid equilibria (VLE) for polymer/solvent systems and swelling equilibria of thermosensitive hydrogel systems using the same energy parameters obtained from LLE calculations. (C) 2010 Elsevier B.V. All rights reserved.