Phosphonic acid and sulfonic acid cation exchange membranes were prepared from copolymers of chloromethylstyrene and divinylbenzene or styrene and divinylbenzene. The electrodialytic transport properties, especially the relative transport number between two cations and the current efficiency, were compared. Multivalent cations permeate the membranes with phosphonic acid groups more slowly than the membranes with sulfonic acid groups. When a mixed salt solution of multivalent cations and sodium ions was electrodialyzed, the current efficiency was low with the phosphonic acid membrane, due to binding of multivalent cations to phosphonic acid groups as confirmed by measuring the water content of the membranes of the alkaline earth cation form. When a cationic charged layer, which prevents permeation of multivalent cations through the membrane, was formed on the surface of the phosphonic acid membrane, the current efficiency increased and the relative transport number of multivalent cations to sodium ions decreased.