organic-inorganic composite mono-valent cation selective membranes (MCSMs) were prepared by sol-gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic segment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical, thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for monovalent cations, ionic diffusion and water transport number. These properties suggested the suitability of MCSMs, especially Si-65%, for electro-separation of Na+ from Ca2+, Mg2+, and Fe3+. The effect of electrolyte solution on the characteristics of the current-voltage (i-v) curve in MCSM was studied based on the concentration polarization. Electro-transport of different ions in terms of plateau length and concentration profiles for different ions in the solution phase, diffusion boundary layer and membrane phase were presented. Information obtained from i-v curve analysis were validated by electrodialysis (ED) experiments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor of different ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability of MCSMs for separating cations. (C) 2009 Published by Elsevier B.V.