Absorbable Organic Halogens (AOX) include a number of substances resulting from the reaction of halogens (fluorine, chlorine, bromine and iodine) with organic molecules, being mainly produced by anthropogenic activities. Their considerable toxicity and environmental persistence demand monitoring in surface waters and effluents. AOX are produced as waste from industrial activities such as chemical washing of clothes, metal degreasing, paper bleaching and water treatment with chlorine compounds. Also, AOX wastes are produced by domestic and agricultural activities, by using disinfectants and pesticides. Once AOX is a sum parameter, it does not discriminate which specific molecules are being analyzed, but it is a good parameter to use in preliminary evaluations of water and effluent quality. This paper presents the results of a monitoring initiative of AOX in public water supply in Southern Brazil. The study area included a region which has considerable impacts on its water resources as a consequence of domestic and industrial pollution and land misuse. The results showed high levels of AOX in treated water (overall average = 0.26 mg.L-1 ), which were higher than those found in surface waters (general media = 0.028 mg.L-1 ), indicating that the formation of these toxic compounds is related especially with the chlorination of water in the process of treatment. Probably, the poor quality of raw water allows the persistence of dissolved organic matter even after the water treatment, when chlorine disinfection resulted in AOX production. Concentrations of AOX in the public water supply indicate health risks to human consumers and environment.