An electrodeionization apparatus and process are provided for removing ions from liquids. Liquid to be purified is passed through depleting compartments containing mixed anion and cation exchange resin beads while a second liquid is passed through concentrating compartments also containing ion exchange resin beads. Ions, under influence of a D.C. potential, pass from the depleting compartments into the concentrating compartments through ion permeable membranes. The beads in the depleting compartments are housed within subcompartments of controlled width and thickness and are retained therein by the ion permeable membranes which are secured to the walls of the subcompartments. Means are provided for reversing the polarity of the D.C. potential to convert the depleting compartments to concentrating compartments and the concentrating compartments to depleting compartments. Means are provided for continuously recovering purified water from the apparatus regardless of the polarity of the D.C. potential and the timing of the polarity reversal cycle.