Abstract In order to give the monovalent cation permselectivity to the cation exchange membrane permanently, polyethyleneimine layer was formed on the membrane surface by acid‐amide bond between polyethyleneimine and the SO 2 Cl groups of a styrene‐divinylbenzene copolymer membrane. After the reaction, the SO 2 Cl groups remaining in the inner part of the membrane were hydrolyzed by immersing the membrane into an aqueous sodium hydroxide solution. The resultant cation exchange membranes were evaluated by electrodialysis of sea water from the point of view of the monovalent cation permselectivity, electric resistance of the membrane, current efficiency, and transport number of sodium ions calculated by membrane potential in connection with reduced viscosity of the various commercial polyethyleneimines, ATR‐IR measurements on the membrane surface suggested that the acid‐amide bond was stable for severe hydrolysis reaction.