Brominated trihalomethanes (Br-THMs) such as CHCl2Br, CHClBr2, and CHBr3 are produced by the reaction of hypobromite with humic acid in the presence of hypochlorite. In the presence of excess NaOCl, addition of NaOBr enhanced the formation of Br-THMs but reduced the formation of CHCl3. The product distribution of THMs was affected by the ratio of [NaOBr]/[NaOCl] and was independent of pH and reaction time. In the presence of excess NaOBr, the yield of CHBr3 only increased linearly with the NaOCl concentration added. However, the other three THMs were hardly produced even though NaOCl concentration was increased up to 0.5 of the [NaOCl]/[NaOBr] molar ratio. Our results suggest that in the process of THM formation, hypochlorite ion reacts effectively with humic acid in the oxidation reaction and hypobromous acid plays a predominant role in the electrophilic substitution when both of hypohalites are present.