The ion exchange demineralizer offers extra advantages by utilizing upflow service with downflow regeneration. With this process, the concern over keeping the resin bed in place while allowing for adequate regeneration is minimized. It allows for lower rinsing rates than other processes by utilizing a rinse recycle step. While prefiltration is important to an ion exchange process, it is less critical on upflow service. The process allows for periodic external backwashing to relieve bed compaction or to remove resin fines.