The results of the study of HAA s formation in the Central Waterworks in Warsaw, Poland are presented. Water was taken from the Vistula River by two methods: infiltration and bank intake, then treated in two parallel water treatment systems using different technologies. Water was analyzed during various steps of treatment and the following parameters were determined: HAA s, COD -Mn, colour, TOC, UV₂₅₄ absorbance and temperature. The removal of organic substances during water treatment was presented. Water was disinfected with chlorine, chlorine dioxide or a mixture of the two. It was found that formation of haloacetic acids depended on water temperature, organic precursors of HAA s and disinfectant type. The study was performed from January 2002 to January 2003. The highest concentration of HAA s was noticed in May and September.