Abstract The chlorine is the reagent of the widest application in disinfection processes of drinking water but it shows the short coming of reacting with the organic substances therein giving trihalomethanes with possible cancerous action. For this reason the tendency is noticed of replacing chlorine by reagents with higher disinfecting properties and lack of final toxic products. The chlorine dioxide could be used to this purpose. A comparative study on the effects obtained by treating the Prut water with chlorine and chlorine dioxide made evident the better results afforded by the last in the oxidative and biocide processes giving a drinking water of higher quality.