Despite its low atmospheric abundance(typically a few ppt),bromine monoxide(BrO)make significant contributions to the destruction of the ozone layer in the atmosphere [1-2].Sakamoto et al.measured the rate constant of C2H5O2(the second large RO2)with Bro being(3.8+1.7)xl0-12 cm3 molecule-1 s-1 [3].Although it has been found that the following channel occurs similarly in the RO2(R=CH3)+BrO reactions [4]: RO2+BrO→dduct→transition state→CH3CHO+HOOBr To our best knowledge that assumed channels are unclear yet.Using QCISD(T)/6-311+G(2df,2p)//B3LYP/6-311+G(2df,2p),Mechanisms and reaction pathways for the C2H5O2+Bro reaction have been investigated.It has been found that the title reaction would take plae on both the singlet and triplet potential energy surfaces(PES).On the singlet PES,the most important products include C2H5OOOBr,C2H5OOBrO,C2H50BrO2,CH3CHO+HOOBr,CH3CHO2+HOBr and CH3CHO+HBrO2.While other products such as CH3CHO+HOOBr,C2H5O+OBrO,C2H5O+ BrOO,and C2H5OBr+O2(1Δ and 3∑)are negligible due to high barriers or unstable formations.A11 the channels on the triplet PES play of no importance under atmospheric conditions due to high barriers.