A physicochemical model is proposed to describe sorption in proton-exchange membranes (PEMs), which can predict the complete isotherm as well as provide a plausible explanation for the long-unresolved phenomenon termed Schroeder's paradox, namely, the difference between the amounts sorbed from a liquid solvent vs. from its saturated vapor. The solvent uptake is governed by the swelling pressure caused within the membrane as a result of stretching of the polymer chains upon solvent uptake, Pi(M), as well as a surface pressure, Pi(sigma), due to the curved vapor-liquid interface of pore liquid. Further, the solvent molecules in the membrane are divided into those that are chemically, or strongly, bound to the acid sites, lambda(i)(C), and others that are free to physically equilibrate between the fluid and the membrane phases, lambda(i)(F). The model predicts the isotherm over the whole range of humidities satisfactorily and also provides a rational explanation for the Schroeder's paradox. (C) 2003 The Electrochemical Society.