A current-voltage characteristic of a Neosepta AMX membrane in a monosodium phosphate solution was investigated experimentally and theoretically. It is shown that the experimental curve has two sections of the overlimiting " plateau" and, accordingly, two limiting current densities. This feature is not observed in the presence of electrolytes, such as sodium chloride. For a better understanding of the phenomenon, a one-dimensional three-layer steady-state transfer model taking into account the possibility of ampholyte species transformation depending on the local pH is developed. Comparison of the calculations with the experimental data and the interpretation of the current-voltage characteristic are made. The first limiting current is due to the vanishing of single-charged phosphate anions in the solution at the membrane surface. The following growth of current density is caused by the protons generated during the transformation of the single-charged phosphate anions (H2PO4-) into the double-charged ones (HPO42-) when entering the membrane. This transformation is due to the increase in pH of the AMX internal solution with increasing current density. The second limiting current is observed when the membrane is almost completely converted into the form of double-charged phosphate anions. In this state the proton flux generated in the H2PO4--HPO42- transformation at the depleted solution/membrane interface approaches its maximum.