A phenomenological theory is provided for water sorption and proton transport in polymer electrolyte membranes (PEMs) as well as in polymer-inorganic nanocomposite membranes (NCPEMs) that not only serves to rationalize the sorption and conductivity behavior of conventional PEMs such as Nafion but also provides a framework for rational design of improved PEMs and NCPEMs. The thermodynamic model, which considers the effect of osmotic pressure on the activity of free water within the membrane pores, predicts the entire sorption isotherm and provides a plausible explanation for the so-called Schroeder's paradox. The transport model incorporates the various mechanisms of proton transport, namely, surface hopping, Grotthuss diffusion, and en masse diffusion. As the design of alternate PEMs suitable for effective proton transport under hot and dry conditions is a key current technological goal, the rational design of NCPEMs for this purpose is considered here in detail on the basis of an extension of the transport model to account for the influence of the inclusion of functional additives in NCPEMs. The results also point to the reason that Nafion is an excellent PEM, because the hydrophobic nature of its backbone induces water away from surface into pore bulk where efficient proton diffusion occurs. (c) 2006 Wiley Periodicals, Inc.