Oxidation of Methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by reaction with hydroxyl radicals. Methyl ethyl sulfide (MES) radicals generated after losing an H atom via OH abstraction subsequently reacts with O-2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH2 center dot SCH2CH3 radical and molecular oxygen are analyzed using quantum Rice-Ramsperger-Kassel (QRRK) theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products and transition states are determined by the CBS-QB3 composite and M062X/6-311+G(2d, p) OFT methods. The reaction of CH2 center dot SCH2CH3 with O-2 forms an energized peroxy adduct center dot OOCH2SCH2CH3 with a calculated well depth of 26.4 kcal/mol at the CBS-Q//M062x/6-311+g(2d,p) levels of theory. Thermochemical properties of reactants, transition states and products obtained under CBS-QB3 level are used for calculation of the thermochemical and kinetic parameters. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the center dot OOCH2SCH2CH3 adduct are important under high pressure and low temperature. At temperatures between above 600-8001< reactions of the chemically activated peroxy adduct become important relative to stabilization under the atmospheric pressure. Two new pathways are observed, in addition to conventional hydrogen atom transfer reactions from the three carbons to the peroxy oxygen radical. One of these new paths is formation of a Criegee intermediate plus CH2 center dot OO. plus CCS center dot. A second new path involves the peroxy oxygen radical addition to the sulfur moiety followed by carbon-sulfur bond cleavage with formation of carbon-oxygen and oxygen-sulfur double bonds: CH2=O + S center dot(=O)CC. These are potentially important new pathways other alkyl-sulfide peroxy radical systems under thermal or combustion conditions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.