A new equation of state (EOS) is established to describe water activities of polymeric membrane/water systems in fuel cell. It is developed based on the modified perturbed hard-sphere-chain (PHSC) EOS by introducing a new perturbation equation that is obtained from the generalized Lennard-Jones (GLJ) potential function based on a statistical-mechanical relationship. Experimentally observed water activities of polymeric electrolyte/water systems are interpreted by the proposed model. The values calculated from the proposed model are in a good agreement with the experimental data for given systems. (c) 2006 Elsevier B.V. All rights reserved.