Abstract The high-frequency resistance of a Nafion® perfluorosulfonic membrane is measured by means of the AC impedance technique. Using a mercury cell, it is possible to follow the variation of the resistance as a function of the water content and of the nature of the cation present in the membrane. In all cases, the number of sorbed co-ions is very low. When only one counter-ion is present within the membrane phase, the results show that the percolation theory is valid only for lower degrees of swelling. For higher water content, the rate constant of the elementary ion transfer reaction of alkaline cations varies in the same way as their hydration state in water. When the membrane contains both sodium ion and proton, the variation of individual mobility of sodium ion with the molar fraction is deduced from the value of the self-diffusion coefficient measured using a radiotracer. In this case, conductivity measurements reveal that the mobility of the proton increases by up to 150% when the number of sodium ions increases in the membrane.