High purity electrodeionization

Electrodeionization apparatus for purifying water includes a cathode, an anode, and a plurality of alternating anion permeable membranes and cation permeable membranes between the cathode and anode that define concentrating and diluting flow channels between adjacent pairs of membranes. Each concentrating flow channel includes a first guard channel adjacent to the anion permeable membrane, a second guard channel adjacent to the cation permeable membrane, and a brine channel between the first and second guard channels. The first and second guard channels have water with lower concentration of dissolved ions than water in the brine channel so as limit parasitic transfer from a concentrating flow channel to a diluting flow channel.