An advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion-exchange membrane is developed considering the effect of the varying ionic composition on the membrane water content. The transport problem is solved numerically for a set of ratios of the diffusion coefficients of the two counterions and 1:1 ion-exchange stoichiometry. The model is used to evaluate the diffusion coefficients of alkali metal cations in the as-received and expanded H- and M-form Nafion(R) (M = Li, Na, K, Rb, Cs) from ion-exchange measurements. Owing to a compensating effect of the electro-osmotic pore fluid flow the initial rates of ion exchange correspond to a fixed water content which, however, is different in H- and M-form membranes. A strong correlation is revealed between the ratio of the membrane to aqueous ion diffusion coefficients and the polymer-phase volume fraction. It is concluded that the polymer phase presents mainly a steric effect without significantly changing the mechanism of transport of alkali metal cations or protons, which resembles that in bulk water. The different behaviors of the as-received and expanded Nafion(R) forms are probably associated with the prevailing cluster- or pore-network morphology respectively.