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In a combined experimental and theoretical study, we show that capacitive charging of porous electrodes in multicomponent electrolytes may lead to the phenomenon of time-dependent ion selectivity of the electrical double layers (EDLs) in the electrodes. This effect is found in experiments on capacitive deionization of water containing NaCl/CaCl₂ mixtures, when the concentration of Na⁺ ions in the water is five times the Ca²⁺-ion concentration. In this experiment, after applying a voltage difference between two porous carbon electrodes, first the majority monovalent Na⁺ cations are preferentially adsorbed in the EDLs, and later, they are gradually replaced by the minority, divalent Ca²⁺ cations. In a process where this ion adsorption step is followed by washing the electrode with freshwater under open-circuit conditions, and subsequent release of the ions while the cell is short-circuited, a product stream is obtained …