The advantages of the rotating membrane disk (RMD) method, which allows the examination of electromembrane system (EMS) polarization under conditions of uniform availability of both surfaces of the membrane, are shown. To obtain accurate information on the values of membrane potential (and thus on the solution concentration near the membrane surfaces), diffusional relaxation in combination with a RMD has been investigated. Reversal regimes are studied while using the RMD. To calibrate the cell with flowing solution, an apparatus with a RMD is used. To determine the selectivity value of ion-exchange membranes under limiting current density conditions, a method is developed whereby the rotating membrane allows a constant value of diffusion layers to be preserved. Different kinds of ion-exchange membrane (CMV, AMV, Neosepta, Flemion, Ionac, Rhone-Poulenc, MC-40, MA-40, MF-4SC) have been studied. it is found that, under limiting current density conditions, the selectivity value P in the majority of the membranes (except the hydrophobic perfluorinated membranes Flemion and MF-4SC) is considerably tower than it would be under equilibrium conditions. The effect on membrane selectivity produced by excess hydrostatic pressure on one side of the membrane disk is noted.