An adequate understanding of the conductivity of polyperfluorosulfonic acid (PFSA) membranes as a function of water content, or relative humidity, and temperature is necessary for an analysis of the functioning of proton-exchange membrane (PEM) fuel cells. Although much work has been done toward elucidating the microstructure and conduction mechanism in PFSA, a satisfactory theoretical model with a minimum of fitted parameters is not yet available. Such a model is developed here for the conduction of protons in hydrated Nafion(R) or like membranes based on the dusty-fluid model for transport and the percolation model for structural aspects. Further, thermodynamics of dissociation of the acid groups in the presence of polar solvents such as water is included. The sorption of solvent from vapor is modeled using a finite-layer Brunauer-Emmett-Teller (BET) model. With the only fitted parameters employed being the BET constants, determined independently, and the ratio of diffusion coefficients representing the interaction of the protonated solvent molecules with solvent and that with the membrane, the model provides excellent correlation with a variety of experimental data.