Abstract: A mixed resin bed deionizing apparatus for removing toxic or mineral substances from fluid passing through a loading vessel containing a mixed bed of cation and anion resins and a regeneration assembly for regenerating the cation and anion resins for return to the loading vessel, the assembly including a separation vessel for separating the cation and anion resins and two "U"-shaped regeneration vessels, one for regenerating cation resins and one for regenerating aniona resins, the resins being admitted to the regeneration vessels in predetermined amounts which are advanced through the regeneration vessels by removing a predetermined amounts of regenerated resin from the discharge end of the vessel, shifting the remaining resin in the vessel to refill the space in the discharge end and adding a corresponding amount of exhausted or loaded resin to the other end, passing a regenerant through the regeneration vessel counter to the direction of movement of the resin through the regeneration vessel and rinsing the