Theoretical approaches to describe the interdiffusion of aqueous solutions of inorganic 1: 1 electrolytes having a common anion across a cation-exchange membrane have been discussed. A new method to characterize the interdiffusion of cations across an MF-4SC sulfonated cation-exchange membrane in terms of the "fine-pore membrane" homogeneous model taking into account the physicochemical properties of the membrane has been proposed. It has been theoretically found that the interdiffusion coefficient of the membrane with respect to a pair of electrolytes is inversely proportional to the difference in the reciprocals of the diffusion coefficients of the cations and depends on the ratio between the fixed charge density and the electrolyte concentration and on the equilibrium distribution coefficients of the cation-anion ionic pairs in the membrane matrix. The adequacy of the proposed approach has been confirmed via comparing theoretical calculations and experimental results on the interdiffusion permeability of the MF-4SC membranes.