Capacitive deionization of water using membrane-electrode assembly was researched. The approach that allows to decrease energy consumptions of water purification was considered. The method provides usage of mosaic membrane containing both cation and anion exchange fragments instead of glass spacer. Counter-ions inside the membrane ensure rather high ionic conductivity even in pure water. Mosaic membranes based on polyethylene (film type) and phenol-formaldehyde (fibrous type) matrices were studied, their electric conductivity and exchange capacity were determined. Deionization in static and dynamic electrochemical cells, which were filled with deionized water and 0.005 M KCl solution respectively, was researched. The mechanism of electric double layer charging inside pores of the electrodes impregnated with pure water has been proposed. Specific energy consumptions for deionization of very diluted solutions are sufficiently lower for the cell containing mosaic membrane than those for the cell with inert glass spacer. Minimum energy consumptions and maximum deionization degree are reached at cell voltage of 1.4 V. The value of specific energy consumptions is 12 Wh mol(-1) for the laboratory cell containing the mosaic membrane, when degree of deionization reaches 50%, cell voltage is 1.4 V, electrode area is 50 cm(2), initial concentration of the KCl solution is 0.005 M.