W. E. Morf's well-known equation for the electric potential of ion-exchange membranes is analyzed in solutions with uni- and divalent counterions. A generalization of this equation for the case of membranes with two sorts of ion exchanger is obtained. The nature of the 'anomalous' response (potential 'dip' and 'hyperfunction') is discussed. These anomalies are shown to arise from transport of the principal counterions from the interior to the surface of the membrane when the mobility of the exchanger ions is low and association between the exchanger ions and the secondary counterions penetrating the membrane is strong.