The singlet potential energy surface for the CSO_2 system was investigated at the hybrid density functional B3LYP and CCSD(T) (single-point) levels with 6-311G~(**) basis set in order to clarify the reaction mechanism of the radical CS with oxygen. It is shown that the C-site of the CS radical attacks the O_2 bond and forms the isomer 4. The entrance step gives out a lot of energy. The isomer 4 undergoes the 1,2 0-atom shift or the 1,2 S-atom shift and ultimately forms the products. This mechanism can excellently express the formation of the P_1(CO + SO), P_2(CO_2+S) and P_3(COS+O).

Key words: AB INITIO CALCULATION, DFT, POTENTIAL ENERGY SURFACES, FREE RADICAL, OXYGEN, CS, REACTION MECHANISM