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Inorganic-organic composites based on the foil and standard RALEX® cation-exchange heterogeneous membranes (Mega a.s., Czech Republic) were prepared by in situ modification with sulfated zirconia (S-ZrO2). The composite membranes were characterized by SEM, TGA, X-ray diffraction, and FTIR spectroscopy. The effect of S-ZrO2 doping on membrane transport properties was studied using measurements of water uptake, ion-exchange capacity, conductivity, cation diffusion, hydrogen permeability, current-voltage characteristics, and membrane specific permselectivity (Ca2+/Na+). The S-ZrO2 incorporation leads to an increase in conductivity and permselectivity of the composite membranes. The proton conductivity of the S-ZrO2-doped foil membrane (0.0316 S/cm at 30°С) is 4 times higher than that of the pristine membrane. The Ca2+/Na+ permselectivity of the standard RALEX® CM membrane doped by S-ZrO2 reaches 3.8 at low current densities. Moreover, the composite membranes retain their selectivity during the long-term tests (> 50 h continuous electrodialysis). The sulfated zirconia doping of heterogeneous membranes demonstrated an excellent separation efficiency that can be used in wastewater treatment, desalination, and related electromembrane separation processes as well as to reduce scaling of electrodialysis modules. [ABSTRACT FROM AUTHOR]