An electrodeionization apparatus is provided for removing ions from liquids. Liquid to be purified is passed through at least two ion depletion compartments containing mixed anion and cation exchange resin beads in a given separation stage. A second liquid is passed through concentration compartments free of ion exchange resin beads. Ions under the influence of D.C. potential pass from the depletion compartments into the concentration compartments through ion permeable membranes. The beads in the depletion 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.