We show a criticality of three water morphological transitions on pore-water transport and proton conductivity in Nafion of a polymer electrolyte membrane fuel cell, addressing its pore-size distribution and the Schroder paradox. The first transition leads to the onset of proton conductivity; the second allows for the onset of the capillary percolation channels and proton conductivity jump at a low water content lambda(im,H2O) = 5. Using this as immobile saturation. the predicted water distribution and cell performance are in reasonable agreement with the available experiments. The third (the paradox, postulating further capillary advancing) bounds the maximum water content on the cathode side of Nafion, which is also supported by the proposed adsorption isobar (thermodynamic equilibrium limit). These transitions appear in the available pore-size experiments which show capillary percolation channel sizes. In addition, the optimal Nafion pore-water content is between the second and third transitions. (C) 2009 The Electrochemical Society. (DOI: 10.1149/1.3186027] All rights reserved.