The quantum Rice-Ramsperger-Kassel (QRRK) theory is used to analyze the reaction between the activated CH2•SCH2CH3 and molecular oxygen to account for further reaction and collisional and deactivation. hydroxyl radicals initiate the oxidation of Methyl ethyl sulfide (CH3SCH2CH3) and MES (methylthioethane) under combustion conditions. The CBS-QB3 composite and M062X/6-311+G(2d, p) DFT methods was used to study the thermochemical properties of reactants, products and transition states. These thermochemical properties are used for the calculations for kinetic and thermochemical parameters. Under high pressure and low temperature, isomerization and stabilization of the •OOCH2SCH2CH3 adduct is of importance. Under atmospheric pressure and at temperatures between above 600 ~ 800 K reactions of the chemically activated peroxy adduct become important relative to stabilization. The reaction between CH2•SCH2CH3 and oxygen molecule (O2) forms an energized peroxy adduct •OOCH2SCH2CH3 with a well depth of 26.4 kcal/mol, calculated at the CBS-Q//M062x/6-311+g(2d,p) levels of theory. Two new pathways are detected, the first new pathway is Criegee intermediate plus CCS• plus CH2•OO• formation. Thew second new pathway is carbon-sulfur bond cleavage with formation of carbon-oxygen and oxygen-sulfur double bonds after peroxy oxygen radical addition to sulfur atom: CH2=O + S•(=O) CC.