||A variety of disinfection byproducts (DBPs) can be formed when chemicals used for disinfecting drinking water react with natural organic matter (NOM) and/or bromide in raw water. The results confirmed that free bromine species reacted with NOM much faster than that of chlorine not only as an efficient substituting agent but also as an effective oxidant. Lower pH favored the formation of total organic halogen (TOX). At the same time, TOX formation was not sensitive to varying bromide levels. Transformation of high molecular weight (MW) precursors to low WM precursors, as indicated by UVA272, resulted in the accumulation of low MW precursors during the first hour of chlorination. TOX in the low MW fractions increased along with the contact time of chlorination/bromination, whereas TOX in the high MW fractions reached the maximum at the contact time of 24 hours and then decreased. Bromination favored the formation of low MW TOX as compared with chlorination. The presence of bromide did not significantly change the observed photolysis rate of total free halogen during the UV/chlorine process. Upon UV irradiation, chlorine is always far more efficient in achieving higher [OH]ss than bromine. The UV/halogen process led to supplementary changes in NOM molecules as compared to dark halogenation, while no significant difference can be found regarding TOX formation between the UV/halogen process and dark halogenation of NOM solutions. Model organic compound studies demonstrated a structure-dependent response to the UV/halogen coexposure process, where significant shift in TOX composition from halogenated aromatic structures to low MW haloacids was observed. Halate formation from the UV/chlorine process was significantly accelerated as compared to the dark chlorination. The bromate formation was governed by the fate of BrO· and BrO2- as decisive intermediates. The scavenging of BrO2- by excessive free chlorine significantly hindered the bromate formation from ·OH-dominated multistep oxidation. The highest bromate formation was observed at alkaline pH under medium-pressure UV lamp due to direct formation of BrO· from OBr-. When the bromide concentration in raw water exceeding 1 mg/L, the UV/chlorine process may induce potential risk of bromate formation exceeding 10 μg/L at a UV dose of 480 mJ/cm2. The corresponding chlorate formation was minor considering the provisional guideline value of 0.7 mg/L.