Polarization phenomena occurring at ion‐exchange membranes have been investigated with a number of commercial membranes of the type used in electrodialysis cells for desalting. Polarization is most marked with very dilute electrolyte solutions and with conditions favoring stagnation of the liquid films near the membrane; in particular, a horizontal membrane with upward direction of migration of counterions minimizes convection. Chronopotentiometric studies with this system have shown that the early stages of concentration polarization conform approximately to Sand's theory. Later, however, complicated phenomena are observed. At high current densities all of the cation‐exchange membranes tested and certain of the anion‐exchange membranes showed rapidly rising resistance, but certain other anion‐exchangers were found capable of carrying high currents continuously, mainly in the form of hydroxyl ions. Experiments with radiotracers indicated that these differences did not arise from co‐ion leakage or electroosmotic streaming. A possible explanation is that ease of passage of H+ or ions through these membranes is dependent on the degree of microheterogeneity, which affects the possibility of transport by the Grotthus mechanism.