A new group contribution model based on a modified double lattice (MDL) theory is developed and applied to describe the ionic conductivities of perfluorosulfonic acid (PFSA) membrane/water systems. The proposed model includes a combinatorial energy contribution that consists of the revised Flory-Huggins entropy of mixing, the van der Waals energy from dispersion, polar force and specific energy contributions. We investigated six groups used for PFSA membranes and changed the configuration of polymer, which was calculated separately. To obtain good mechanical properties of PFSA membranes, the specific polymer configuration was optimized. As a result, shorter CF2CF2 groups in polymer backbone chain and in the side chain lead to a higher ionic conductivity, as does a single CF2CFCF3O group in the side chain. Quantitative description according to the proposed model is in good agreement with experimentally observed physical property for a given system. (C) 2009 Elsevier Ltd. All rights reserved.