Electric conductivity of homogeneous cation-exchange (CMX) and anion-exchange (AMX, AX) membranes in the salt solutions of some organic (lysine hydrogen chloride and potassium hydrogen tartrate (KHT)) and inorganic (sodium dihydrogen phosphate) ampholytes was measured as a function of their concentration. In all cases, an increase in membrane conductivity with diluting external solution was observed at concentrations < 0.04 mol L-1. This trend is not observed in the presence of strong electrolytes, such as NaCl. The effect is explained by a shift in ionic equilibriums caused by decreasing external concentration: increasing Donnan exclusion of H (broken vertical bar) ions from an anion-exchange membrane (AEM) as co-ions and increasing fraction of OH- ions in the external solution when it is diluted under a constant pH. This leads to an increase in pH of the internal charged solution in the membrane gel phase and transformation of a part of singly charged ampholyte anions (e.g. H2PO4-) into doubly charged ones (HPO42-). The conductivity increases when one doubly charged counterion replaces two singly charged ones. The mathematical simulation of membrane conductivity concentration dependence provides a good accordance with experimental data The effect was also simulated by measuring and calculating the conductivity of mixed KHT-K2T external solutions as a function of the equivalent fraction of the tartrate ion (T2-) under condition that the total equivalent electrolyte concentration was maintained constant. The shape of the conductivity vs. concentration curve strongly depends on the ampholyte nature, namely, on the (pK(2)-pK(1))/2 value. (C) 2015 Elsevier B.V. All rights reserved.