Membrane selectively permeable to monovalent cations and process for producing the cation exchange.

A membrane selectively permeable cation exchange monovalent cations, characterized by having, on at least one side of the cation exchange membrane, a treated surface to contact cations molecular top of a homopolymer or copolymer of allylamine having a weight average molecular of at least 5,000 in the presence of an oxyacid anion formed from nitric acid, nitrous acid, sulfuric acid, sulfurous acid, pyrosulfuric acid, carbonic acid, phosphoric acid, silicic acid or a salt thereof,