The problem of formation of the toxic carcinogenic substances in the process of the water treatment during disinfection of natural waters containing organic pollution is actual both in Russia and in foreign countries. There is an information about the negative effects of trihalomethanes (THM) on the human organism. The examples shows that, the concentration of trihalomethanes in the drinking water exceeds the standards several times in water treatment according to the traditional technology with the use of chlorine. The aim of the work was to study the dynamics of THM changes in the stages of purification in the comparison with the water quality, the reagent treatment modes and technological parameters to assess the barrier role of existing water supply stations. It was found that the standard degree of the purification achieves with chromaticity, turbidity, permanganate oxidation, bacteriological parameters. At the same time, the prechlorination of water with a temperature of 1 ° C and a chromaticity of 67-71 °C leads to the formation of chloroform in the amount of 40-61 µg/l, and postchlorination at the final stage of the full cycle contributes to an increase in the concentration to 192-215 µg/l. The new data were the basis for the development of the improved water purification technology and measures for the modernization of the water supply stations.