The selectivity mechanism of transport of Na+, Ca2+ and Mg2+ through commercial monovalent-cation permselective membranes is investigated in batch electrodialysis experiments with synthetic salt solutions containing monovalent and divalent cations. The role of hydration energy, steric effect, kinetic effect as well as effects of permselectivity of cation exchange membrane has been elucidated with electrodialysis of single solutions (NaCl, CaCl2, MgCl2). The mechanism of interferences is investigated in (Na+/Ca2+, Na+/Mg2+, Ca2+/Mg2+ and Na+/Ca2+/Mg2+) mixtures.