Abstract The self-diffusion coefficient of an ion and the membrane permeability were studied with a system consisting of two identical electrolyte solutions separated by a cation exchange membrane. The membrane permeability to cation was estimated from the membrane conductance data. The result indicated that the membrane permeability increased with a decrease in electrolyte concentration. The self-diffusion coefficients and the concentrations of Na+, Cs+, and Ca2+ in the membrane were also measured by the radiotracer method. At concentrations less than 0.1 M, the self-diffusion coefficients more or less increased with increasing concentrations, while the concentrations within membrane remained almost constant. The membrane permeability to ion estimated from the conductance data was in excellent agreement with that calculated by the relation in which the membrane permeability is given as the self-diffusion coefficient times the partition coefficient divided by the membrane thickness.