Three perfluorosulfonic acid membranes, Nafion 120, Nafion 117 and Dow, pretreated in water at 100-degrees-C were characterized by conductivity measurements in the temperature range of 20-90-degrees-C and by Small-Angle X-Ray Scattering (SAXS) at room temperature. The results demonstrate that conductivity is determined by the apparent proton equivalent conductivity and not the proton volume concentration. The apparent equivalent conductivities increase in the sequence Nafion 120 < Nafion 117 < Dow. SAXS reveals the same average cluster size of 5.3 nm for all three membranes but different cluster densities increasing in the sequence : Nafion 120 < Nafion 117 < Dow. Therefore, the proton equivalent conductivity is determined by the cluster density with the polymer network.