A method has been developed which permits one to determine the electrolyte permeation velocities and the membrane system permeability from measurements of the membrane potential as a function of time. The method has been applied to a cation-exchange membrane separating two aqueous KCl solutions, at the same pressure and temperature, but of different concentrations. The experiments have been carried out in two concentration ranges and at different solution stirring rates. The obtained results showed that the membrane system permeability depends notably on the stirring rate, this dependence being greater for the highest concentration range. The intrinsic permeability of the membrane was determined from considerations about the concentration polarization effect, its value being greater in the lower concentration range.