The reaction mechanism of OBrO with OH has been studied using the B3LYP/6-311+G(d,p) and the high-level electron-correlation CCSD(T)/6-311+G(d,p) at single-point. The results show that the title reaction could probably proceed by four possible schemes, generating HOBr+O2, HBr+O3, BrO+HO2 and HOBrO2 products, respec-tively. The main channel is the one to yield HOBr+O2. The whole reaction involves the formation of three-membered, four-membered and five-membered rings, followed by the complicated processes of association, H-shift, Br-shift and dissociation. All routes are exothermic. The reaction mechanism of OBrO with OH has been studied using the B3LYP / 6-311 + G (d, p) and the high-level electron- correlation CCSD (T) / 6-311 + G (d, p) at single- point. The results show that the title reaction could probably proceed by four possible schemes, generating HOBr + O2, HBr + O3, BrO + HO2 and HOBrO2 products, respec- tively. The main channel is the one to yield HOBr + O2. whole reaction involves the formation of three-membered, four-membered and five-membered rings, followed by the complicated processes of association, H-shift, Br-shift and dissociation. All routes are exothermic.