Deionization of water by a small singe-body device with four-stage ion-exchange processes for production of deionized water for laboratory analyses (according to BDS ISO 3696 and BS 3978) or small factories was studied. Four ion-exchange processes were performed in a single filter body: cation exchange by weak acid cation exchanger Lewatit CNP 80; cation exchange by strong acid cation exchanger Lewatit S 100; anion exchange by weak base anion exchanger Lewatit MB 64; anion exchange by strong base anion exchanger Dowex SBR. Conventional water treatment ion exchange plants use the same four processes but in more than one filtration body (minimum two). Besides, they need at least 12 valves for water and regeneration flows redistribution and quite a large space for technological localization and service. The small water treatment ion exchange device developed is compact compared to the conventional ones and has only 4 valves.