Abstract: In the course of demineralization of electrolyte solutions by ion exchange membrane electrodialysis, the ionic concentration in the boundary layer formed on the desalting surface of the membrane decreases owing to the concentration polarization. This phenomenon occurs because of the difference of transport number of counter-ions between in the solution and in the membrane. At limiting current density, the ionic concentration at the solution/membrane interface reaches zero and then water dissociation (water splitting) occurs at over limiting current density. The water dissociation is noticed in an electrodialysis process because it decreases current efficiency and gives rise to scale troubles. Further it attracts an attention in a bipolar membrane electrodialysis process or in an electro-deionization process because it increases current efficiency. The water dissociation reaction is first observed by Kressman and Tye (1956) and Frilette (1956). Rosenberg and Tirrel (1957) observed current versus pH relationship due to the water dissociation reaction in a NaCl solution and found that the reaction is suppressed on the cation exchange membrane.