The formation of trihalomethanes (THMs) during chlorine (Cl2) and chlorine dioxide (ClO2) treatment was investigated. Water samples were collected from three drinking water sources, namely, Büyükçekmece Lake water (BLW) in Istanbul City, Kızılcapınar Lake water (KLW), and Ulutan Lake water (ULW) in Zonguldak City, Turkey. The results of the study show that Cl2 treatment forms more THMs in all three water sources compared to ClO2treatment. Further, due to the Cl2treatment, the maximum THMs concentrations were observed in BLW (121.15 μg/dm3) samples, followed by KLW (97.26 μg/dm3) and ULW(88.52 μg/dm3) samples within the reaction time of 24 h for 5 mg/ dm3 of Cl2dose. However, it was found that the concentrations of THMs formed at three water sources with applied ClO2treatment under the same conditions were significantly reduced. As a result of the ClO2treatment at the end of the reaction time of 24h, THMs concentrations formed in BLW, KLW, and ULW were recorded as 30.26, 16.53, and 17.71 μg/ dm3, respectively. On chlorination, chloroform (CFM) was found the dominant THM species in all water sources. All THM species contents ranged from 1.98 μg/dm3 to 11.23 μg/ dm3 and the highest level of dibromochloromethane (BDCFM) was observed as the major THM species among all species in BLW due to the ClO2treatment. Also, the formation of inorganic DBPs such as chlorate (ClO3–) and chlorite ClO2–) was evaluated during Cl2oxidation. The levels ClO2– formed due to the ClO2 oxidation were higher than those of ClO3– levels for BLW, KLW, and ULW samples and varied from 19 to 55%, and from 37 to 60% of the applied ClO2 doses (2–10 mg/ dm3), respectively. On the other hand, ClO3– levels varied between 5 and 9% and 2 and 6% of the applied ClO2concentration for the KLW and ULW samples, respectively.