Separation of divalent cations from binary electrolyte solutions with protons was achieved by electrotransport through modified cation-exchange membranes (CEMs). The CEM modification was carried out by adsorption of polyethyleneimine on the membrane surface. Without any applied driving force, both modified and non-modified membranes exhibited a higher affinity for divalent cations. Under an electrical field, the transport number of the divalent cation decreased significantly. The apparent rate constants of the penetration of the divalent cations into the membrane were calculated on the basis of unidirectional fluxes and concentration profiles of cations within the anodic unstirred layer. The unidirectional fluxes were measured by means of the radiotracer technique. The concentration profiles were determined by numerical resolution of the Nernst-Planck electrodiffusion equations coupled with the Poisson equation for current densities lower than the limiting value.