Seasonal variation in concentrations of two different disinfection by-product groups, trihalomethanes (THMs) and haloacetonitriles (HANs), was investigated in tap water samples collected from five sampling points (one groundwater and four surface water sources) in İzmir, Turkey. Estimates of previously published carcinogenic and noncarcinogenic risks through oral exposure to THMs were re-evaluated using a probabilistic approach that took the seasonal concentration variation into account. Chloroform, bromoform, dibromochloromethane and dichloroacetonitrile were the most frequently detected compounds. Among these, chloroform was detected with the highest concentrations ranging from 0.03 to 98.4 µg/L. In tap water, at the groundwater supplied sampling point, brominated species, bromoform and dibromoacetonitrile, were detected at the highest levels most probably due to bromide ion intrusion from seawater. The highest total THM and total HAN concentrations were detected in spring while the lowest in summer and fall. The annual average total THM concentration measured at one of the surface water supplied sampling points exceeded the USEPA's limit of 80 µg/L. While all non-carcinogenic risks due to exposure to THMs in İzmir drinking water were negligible, carcinogenic risk levels associated with bromodichloromethane and dibromochloromethane were higher than one in million. IntroductionChlorination of drinking water leads to formation of disinfection by-products (DBPs) which may create adverse health effects on human beings. The DBPs are formed as a result of reactions between the precursor materials (natural organic matter and bromide ion) and aqueous forms of the disinfectants. The major groups of DBPs are trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), and halogenated ketones (HKs). THMs include four species; chloroform (CF), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform (BF). Epidemiological studies have suggested that exposure to DBPs increases the risk of bladder, colon-rectum, leukemia, stomach and rectal cancers as well as abortion, low birth weight, and birth defects (IARC, 1991;Calderon, 2000;Villanueva et al., 2004). The formation of DBPs is affected by several factors including water temperature and pH, nature and concentration of the