A micromechanics model is developed to predict Young's modulus of perfluorosulfonic acid (PFSA) membranes at various temperatures and water contents. The morphology of PFSA membranes is characterized by a two-phase structure, where hydrophilic clusters expand to hold water molecules during swelling, whereas the hydrophobic polymer network maintains the structural stability. A representative volume element (RVE) is proposed based on the descriptions for the nanostructure of PFSA membranes available in the literature. On the basis of mechanics model, we estimate Young's modulus in tension of PFSA membranes as a function of water volume fraction for various temperatures. The results show that inclusion of water-filled cylindrical channels connecting the spherical clusters yields significantly better prediction than considering spherical clusters alone. Comparison of these results with the existing experimental data shows good agreement over a range of temperatures. This study takes into account current understanding of the nanostructure of these materials and incorporates it into a micromechanics model that agrees well with the experimental results. (c) 2008 Wiley Periodicals, Inc. J Polyrn Sci Part B: Polyrn Phys 46: 2404-2417,2008