The exchange kinetics of Cs+, Cu2+, and Eu3+ ions in Nafion-117 were measured at different HI ion concentrations in equilibrating salt solutions. The exchange kinetics were found to be independent of the H+ ion concentration in equilibrating salt solutions over the pH range of 5-2. The exchange kinetics were therefore used to evaluate self-diffusion coefficients of the metal ions using an analytical solution of Fick's second law. The self-diffusion coefficient of Cs+ in Nafion-117 was found to be in agreement with the value obtained in our earlier work (1.94 x 10(-7) cm(2) s(-1)) based on the isotopic-exchange method in the absence of HI ions. This further confirmed that the exchange process in the Nafion-117 membrane represents the pure isotopic exchange in the said pH range. The self-diffusion coefficients of Cu2+ and Eu3+ were found to be 2.9 x 10(-7) and 4.5 x 10(-8) cm(2) s(-1), respectively. The smaller self-diffusion coefficient of Eu3+ reflects its increased electrostatic interaction with the membrane matrix. The kinetics of forward (H-(m)(+) = M-(s)(n+)) and reverse (M-(m)(n+) = H-(s)(+)) exchanges of metal ions (Cs+, Cu2+, and Eu3+) in the Nafion-117 membrane were also studied. The Nernst-Planck theory was examined to predict the kinetics of these exchanges. The theory was found to be reasonably accurate for predicting the kinetics of the reverse ion exchange but completely failed for the forward ion exchange. Contrary to the prediction of the theory, the kinetics of forward exchanges of Cs+, Cu2+, and Eu3+ with H+ ions in the Nafion- 117 membrane were found to be independent of the metal ions. This shows that the self-diffusion coefficients of the counterions in Nafion- 117 can be obtained only from the analysis of reverse exchange kinetics based on the Nernst-Planck equation.