The standard porosimetry method (SPM) was used to study capillary porous and sorption properties of Nafionm proton-exchange membranes. Pore volume-size and surface-size distributions were recorded together with water distributions as a function of capillary pressure and desorption isotherms. The measurements were performed at 20 and 80 C; only a small temperature effect was found for the porous volume structure. Using these data, the charge density at the inner pore surfaces was evaluated. The estimate suggested only a small degree of diffuseness of the proton double layer at the inner pore surface.