A new group-contribution model based on the modified double-lattice theory is developed and applied to describe vapor-liquid equilibria of solid polymer electrolyte/water systems. 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, polar force and the specific energy contribution. Quantitative description according to the proposed model is in good agreement with experimentally observed water activities of polymer electrolyte fuel cell systems.