Flow rates of co-ions across membranes (Cl- and SO42 Ions across Neosepta CMX cation exchange membranes, K+ and Mg2+ ions across Neosepta AMX anion exchange membranes) in the title electrodialytic metathesis have been determined. It was established that the flow of co-ions causes contamination of all solutions flowing through the electrodialyzer. For instance, Cl- ions penetrate into anolyte (K2SO4 solution), chlorine is found among anode reaction products and deteriorates operating conditions and induces corrosive processes. A quantitative contamination assessment of two products (potassium sulphate and sodium chloride) was carried out.