The gas-phase reaction of YS + (1 Σ + , 3 Φ) with an S-transfer reagent (COS), YS + +COS→YS + 2 + CO, was studied using density functional theory at the B3LYP/6-311 + G* level. Four parallel reaction pathways were identified on both the ground-and excited-state surfaces. The mechanisms and the geometrical change trends on the different surfaces are quite different, except in the case of one reaction channel. The experimentally observed endothermic feature of the formation of YS + 2 can be attributed to three reaction paths, A, B, and C, with calculation barriers of 28.3, 140.5, and 120.2 kJ•mol-1 , respectively, on the ground singlet surface. Our calculation results show that the title reaction has no two-state reactivity and the exothermic feature of the YS + 2 cross-section observed in the experiments is attributed to reaction of the residual excited-state of YS + in the reactants.