In spite of the potential importance of the HCS radical in both combustion and interstellar processes, its chemical reactivity has not been tackled previously. In the present paper, the oxidation reaction of the HCS radical is theoretically investigated for the first time at the CCSD(T)/6-311++G(3df,2p)//BH&HLYP/6-311++G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels. It is shown that the most feasible pathway is the O2 addition to the HCS radical forming the intermediate SC(H)OO which can undergo a subsequent O-extrusion leading to SC(H)O + ³O. This features an indirect O-transfer mechanism with the overall barrier of 4.4 and 3.5 kcal mol^{â 1}, respectively, at the two levels. However, formation of the H-transfer product CS + HO2 is kinetically much less feasible, i.e., the direct mechanism has barriers of 14.3 and 8.7 kcal mol^{â 1}, whereas the indirect mechanism has barriers of 12.6 and 10.7 kcal mol^{â 1}, respectively. This result is in sharp contrast to the analogous HCO + O2 reaction, where the direct (with a barrier of 2.98 kcal mol^{â 1}) and indirect (2.26 kcal mol^{â 1}) H-transfer processes are highly competitive over the indirect O-transfer process (the least endothermicity is 19.9 kcal mol^{â 1}). The possible explanations and implications of the present results are provided.