A new group-contribution model based on the modified double-lattice theory considering chain-length dependence of the universal parameters was developed and applied to describe the vapor-liquid equilibria of polymer solutions. The proposed model includes the combinatorial energy contribution that is responsible for the revised Flory-Huggins entropy of mixing, the van der Waals energy contribution from dispersion and polar forces, and the specific energy contribution from hydrogen bonding. The quantitative description according to the proposed model was in good agreement with experimentally observed solvent activities of polymer solutions. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2634-2640, 2008