The elimination of contaminants in pool water through the “ozone – activated carbon process” is done stepwise. Sieving, flocculation and filtration steps are followed by ozonation. According to the German standards for the Treatment of Swimming Pool Water, the toxic ozone has to be removed from the water by filtration through an activated carbon layer before the water is brought back into the swimming pool. The treatment step is followed by chlorination for disinfection. In the ozone process described and in most of its modifications the time available for the reaction of the ozone is very short, usually 1.5 up to 3 min. First results with ozonation of model compounds, such as urea, creatinine, amino acids, etc., have shown–that the reaction rates are rather slow. Consequently, a longer reaction time leads to an improved decontamination of the swimming pool water. The efficiency of the ozonation process with longer reaction times followed by slower filtration has been proven in a research project, e.g. by experiments with the so–called “Ozone Combi BlockR– Process”. The potassium permanganate consumption and the combined chlorine as well as the formation potential of haloforms can be reduced effectively by elongated contact timeof ozone.