Data as in Figure 11 exchange reaction at a membrane permeable to the exchanging ions, Rg in fact represents the sum of two resistances in series, one related to the kinetics of the ion exchange reaction and the second representing the mobility of the counter ions in the membrane. Where the glass membrane is coated by a surface-hydrolyzed film, the equivalent circuit has, in addition to that described above, a series Warburg diffusional impedance shunted by a resistance. It is, perhaps, surprising that under an applied electric field the counter ions show any diffusion at all as they are the major charge carrier present in solution. The large resistance in parallel with the Warburg impedance is interpreted as representing the effects of electromigration through 'pores' in the surface film.The results of the current step measurements of potentialtime curves are compatible with the equivalent circuit model obtained by impedance measurements. In the absence of a