Multicomponent diffusion studies were carried out in a poly(perfluorosulfonated) ion-exchange membrane, Nafion-117, using two cations, namely Na+ and Cs+, having widely different self-diffusivity and selectivity. Radiotracers were used to follow the time dependent concentration of the competing ions in the membrane. The studies were carried out at three different composition of the equilibrating solution. At all composition of the equilibrating solutions, the Na+ ion concentration was found to pass through a maximum, while the Cs+ ion concentration continuously increased till equilibrium is reached. This could be explained as being due to higher diffusivity but lower selectivity of Na+ ions than Cs+ ions. An attempt was made to reproduce the time profile of the competitive diffusion for both the ions using the modified form of Maxwell-Stefan formalism. The observed trend could be qualitatively explained using this approach. (C) 2008 Elsevier B.V. All rights reserved.