As the rates of ion transport in membrane and electrolytic solution become. comparable in membrane electrodialysis, the separation efficiency may be limited by the capability of ion exchange of the membrane. An experimental study of current vs. voltage characteristics of dilute electrolytic solutions (KCl, NaCl, and LiCl) was carried out by a laboratory scale membrane electrodialysis unit to show this phenomenon. The observed current vs. voltage characteristics show that in a low electrical field (0 less than or equal to 5 E less than or equal to 100 volt/m), there is a critical change in the apparent conductance of the system with the increased initial ion concentration. This critical phenomenon suggests that in a low electrical field the ion transport is limited by the fate of ion exchange within the membrane. The sequence of these critical phenomena for the solutions of KCl, NaCl, and LiCl indicates that the ion exchange limit may be related to the ion's size as well.