Abstract: Alkali metal hydroxide solutions are purified and concentrated by electrolysis of such solutions in a hybrid cell comprising an anode compartment and a central compartment separated from the anode compartment by a cation permeable diffusion barrier, and a cathode compartment in flow communication with the central compartment and separated from the central compartment by a diaphragm. To enable operation, gaseous hydrogen is supplied to the anode, oxygen to the cathode, an aqueous solution of at least one alkali metal hydroxide to the anode compartment, and an aqueous media receptive to alkali metal ions to the central compartment. A plurality of the hybrid cells may be operated in hydrodynamic series.