The electrical responses of two interpolymer ion-exchange (anion and cation) membranes have been investigated by chronopotentiometry in dilute solutions of NaCl, KCI, and HCl as strong electrolytes and a 75:25 mixture of(1 mM) RCOOH-RCOONa, where R = -H, -CH3, -CH2CH3, -CH2CH2CH3, and -CH(CH3)(2) as weak electrolytes. The behavior of both of these membranes was nearly ideal in strong electrolytes. The performance of a cation membrane in weak electrolytes R = -H and -CH3 was as good as in NaCl/KCl and less satisfactory in others. On the other hand, the performance of an anion membrane in electrolytes R = -H and -CH; was nearly identical to that in NaCl/KCl. but it steadily decreased as the length and size of the alkyl (-R) chain increased. The poor performance of the anion membrane in higher homologous carboxylates with R = -CH2CH3, -CH2CH2CH3, and -CH(CH3)(2) has been attributed to fouling of the surface by the RCOO- groups next to the membrane in the interfacial zone on the diluate side and/or the choking of ion flow channels by these carboxylate ions on account of a long nonconductive alkyl chain on them. The parameters such as the potential drop across the membrane at the outset of each experiment, I tau(1/2), the permselectivity, and the transference number for both types of membranes in strong as well as in weak electrolytes were measured and compared, The transport across the anion membrane of OH-ions from either water autodissociation or hydrolysis step of RCCO- groups at the anion membrane surface was considered in both strong and weak electrolytes at the above optimum current densities. The fouled anion membrane was regenerated by rinsing with distilled water and equilibrating it with a strong electrolyte solution.