It is common practice in electrodialysis demineralization of calcium hydrocarbonate natural waters to restrict the current density by the limiting diffusion one. Exceeding this value causes the irreversible water dissociation at the interface of the anion-exchange membrane and solution and selective transfer of the hydroxyl ions through the anion-exchange membrane into concentrating cells. These hydroxyl ions accumulated in the diffusion boundary layer at the anion-exchange membrane may interact with magnesium, calcium and hydrocarbonate ions to form magnesium hydroxide and calcium carbonate. A method preventing scaling on the membranes is suggested in this article.