Current-voltage characteristics (CVCs) of a Neosepta AMX membrane are studied in NaH2PO4 (pH = 4.7, C = 0.02 mol/L) and KC4H5O6 (pH = 3.6, C = 0.02 mol/L) solutions. It is shown that in the case of NaH2PO4 there are two plateaus in the CVC, which correspond to achievement of the first, ilim(I), and second, ilim(II), limiting currents. i(lim)(I) occurs when the NaH2PO4 salt diffusion to the membrane surface is saturated, ilim(II) refers to the saturation of the proton flux when the membrane is almost completely converted into the HPO42- form. In the case of potassium hydrotartrate, there is no state corresponding to ilim(I). The difference in CVC for two ampholytes is due to the difference in the ratio between the dissociation constants related to the first (K-a1) and second (K-a2) steps of ampholyte dissociation. In the case of NaH2PO4, where pK(a1) and pK(a2) differ greatly, a solution of this salt contains nearly exclusively the singly charged phosphate form. However, a KC4H5O6 solution contains, together with the C4H5O6- anion, about 15% of the doubly charged tartrates, which take part in charge transfer. Approximate analytical expressions are obtained for i(lim)(I), and i(lim)(II) in the case of monosodium hydrophospate solution. Their application gives the ilim(I), and ilim(II) values, which are in good agreement with the experiment and with numerical calculations using a mathematical model based on the Nernst-Planck equation, the electroneutrality condition, and the local chemical equilibrium condition in the membrane and two adjacent diffusion boundary layers. (C) 2018 Elsevier Ltd. All rights reserved.