In this article, the mechanism of the reaction between the atmospheric CH3SCH2 radical and O-2 molecule was studied using an ab initio correlated level of theory. We have performed geometry optimizations at the UMP2/6-31G(d) level of theory and used the additivity approximation to obtain the activation energy at the UCCSD(T)/6-311+G(2df,2p) level. The obtained activation energy is -3.31 kcal/mol. The CH3SCH2O2 radical has three possible conformations, named P1, P2, and P3. However, the conformer formed in the addition reaction, P1, is not the most stable. The transition states to interconvertion between the three conformers were optimized and the rate constants for these reactions were calculated, as well as the exothermicities of each one. These values allowed us to conclude that, in the equilibrium, the distribution of these conformers will be 71% of P2 and 29% of P1. We have used this distribution to obtain a weight average to the exothermicity of the reaction between the CH3SCH2 radical and O-2, which is -26.19 kcal/mol. The heat of formation for the mixture of the two conformers of the CH3SCH2O2 radical was also determined as 6.51 kcal/mol. The reaction free energy was calculated as -15.68 kcal/mol, considering that distribution.