The ion-exchange isotherm of sodium and calcium ions is drawn for the Nafion(R) 117 membrane in contact with [ NaCl] + 2 [CaCl2] = 0.1 M. The self-diffusion coefficients of sodium and calcium ions, obtained from steady-state self-diffusion ionic fluxes by a radiotracer method are independent of the ionic composition of the membrane phase. They are compared to those obtained from high frequency electrical resistance by means of the ac impedance technique. The values of electric conductivity of the membrane can be derived from a simple additivity law.