A cation-exchange membrane has been modified by fixation of polyethyleneimine on its surface. This fixation was carried out under an electric field effect, thus it is called electro-adsorption. The polycation formed in an acidic medium migrated toward the membrane and a charged layer was deposited on the surface, and the selectivity towards divalent ions decreased, yielding to the increase of the proton transfer. When the amount of adsorbed PEI increased, the electrical resistance of the membrane increased and the transport number of zinc decreased. However, suitable conditions like pH, current density and electro-adsorption time were controlled in the order to obtain membranes with better selectivity and low electrical resistance.