The CH3S center dot + O-2 reaction system is considered an important process in atmospheric chemistry and in combustion as a pathway for the exothermic conversion of methane-thiyl radical, CH3S center dot. Several density functional and ab initio computational methods are used in this study to determine thermochemical parameters, reaction paths, and kinetic barriers in the CH3S center dot + O-2 reaction system. The data are also used to evaluate feasibility of the DFT methods for higher molecular weight oxy-sulfur hydrocarbons, where sulfur presents added complexity from its many valence states. The methods include: B3LYP/6-311++G(d,p), B3LYP/6311++G(3df,2p), CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p), B3P86/6-311G(2d,2p)//B3P86/6-31G(d), B3PW91/6-311++G(3df,2p), G3MP2, and CBS-QB3. The well depth for the CH3S center dot+O-3(2) reaction to the syn-CH3SOO center dot adduct is found to be 9.7 kcal/mol. Low barrier exit channels from the syn-CH3SOO center dot adduct include: CH2S + HO2, (TS6, E-a is 12.5 kcal/mol), CH3 + SO2 via CH3SO2 (TS2', E-a is 17.8) and CH3SO + O (TS17, E-a is 24.7) where the activation energy is relative to the syn-CH3SOO center dot stabilized adduct. The transition state (TS5) for formation of the CH3SOO adduct from CH3S center dot + O-2 and the reverse dissociation of CH3SOO to CH3S center dot + O-2 is relatively tight compared to typical association and simple bond dissociation reactions; this is a result of the very weak interaction. Reverse reaction is the dominant dissociation path due to enthalpy and entropy considerations. The rate constants from the chemical activation reaction and from the stabilized adduct to these products are estimated as functions of temperature and pressure. Our forward rate constant and CH3S loss profile are in agreement with the experiments under similar conditions. Of the methods above, the G3MP2 and CBS-QB3 composite methods are recommended for thermochemical determinations on these carbon-sulfur-oxygen systems, when they are feasible.