A local composition model based on the lattice theory and two-fluid theory, considering the excess heat capacity, which is a modified Wilson equation, is developed for the excess Gibbs energy of aqueous polymer solutions. The model represents a synergistic combination of the excess entropy for mixing molecules of different sizes and the temperature dependent residual contribution, which combines the attractive interactions between solvent molecules and the segments with the contribution of the excess heat capacity. The results of the extrapolation with respect to molecular weight of phase equilibrium in aqueous polymer solutions with this model are very satisfactory, with only two adjustable parameters. (C) 2003 Elsevier B.V. All rights reserved.