The ion-exchange kinetics of A(sol)(z+) ? Na-mem(+) (A(z+) = Cs+/Ba2+/Eu3+) in Nafion-117 have been measured using non-stationary radiotracer technique for trace concentrations of A(z+) ions in external solution. A method based on non-steady state Nernst-Planck approach has been developed and used to fit the experimental ion-exchange profiles to obtain the diffusion coefficients (DCs) of these ions. The DCs, thus obtained, have been found to be appreciably lower than the literature-reported self-diffusion coefficients, indicating slower rate of ion-exchange. The results show that membrane DCs of these ions are modified by bulk electrolyte solution.