AbstractThe ion separation by ion‐exchange membranes appears to be of interest as an energy‐saving, continuous and economical process. The separation of ions of the same valency can only be achieved through membranes which have specific interaction with these ions. Ions of different valencies can, however, be separated thanks to the fixed charges in the membrane. For membranes with low hydrodynamic permeability the separation process can be described by the Nernst‐Planck‐Equations. Exact solutions of these equations exist in parametric form. Their numerical evaluation is easily accomplished with the present‐day programmable calculators. — The analysis shows that at relatively high fields coions are more effectively separated then counter ions, although the separation of the latter is more economical in most cases.