The sorption kinetic and equilibrium data obtained at different water activities allowed us to determine the water sorption isotherms and mean diffusion coefficient in the Nafion(R) membrane. The water sorption isotherm is reminiscent of systems in which there is water clustering at high water activities, with a marked Langmuir sorption at low water activity for the acid form of the membrane. The variation of the diffusion coefficient with the water activity showed a maximum value whose coordinates depend on the counter-cation nature. The smaller the counter-cation, i.e. the larger the counter-cation hydration energy, the higher the diffusion coefficient and the lower the water activity at the maximum point.