With the application of chlorine gas in drinking water treatment processing after the turn of this Century, great success was achieved. With the advance of analytical techniques in the field of trace matter, Rook (1)as well as Bellar, Lichtenberg and Kroner (2) succeeded in 1974 to establish the presence of chloroform and other chlorinated compounds in treated drinking water, namely as the result of chlorination. During tests to minimize the problems of chloroform, of chlorite and chlorate by the simultaneous addition of chlorine andchlorine dioxide, it was discovered that already a small dosage of chlorine dioxide was sufficient to reduce drastically the formation of chloroform andother trihalomethanes. The aimof future treatment techniques in Zurich is the preoxidation with ozone in place of chlorine and chlorine dioxide. Considering the mode of effect of the various oxidizing agents and the longer flow time through the biological activated carbon filters (asa result of the ozonation), ozone is – according to studies made by Grob and the Zurich Water Supply with ozonized Lake Zurich water – the most economical oxidation process with the best effect, i.e., without formation of toxic or carcinogenic substances. The pre– and/or intermediate oxidation with ozone surely will be the right way in the foreseeable future.