HO2 and CH3 are major intermediate species presented during the oxidation of natural gas at intermediate temperatures and high pressures. Previous theoretical calculations have identified several product channels for HO2 and CH3 reactions, with CH3 + HO2 - CH3O + OH and CH3 + HO2 -> CH4 + O-2 being the dominant reaction pathways. Both reaction pathways play an important role in the kinetics of CH4 oxidation as CH3 + HO2 CH3O + OH is a chain-branching reaction whereas CH3 + HO2 CH, + O-2 a chain termination reaction. H2O2/CH4/Ar mixtures were shock-heated to a temperature between 1054 and 1249 K near 3.5 atm to initiate the reaction. OH radicals yielded from H2O2 thermal decomposition react with H2O2 and CH4 respectively to produce HO2 and CH3 in the reacting system. Using laser absorption spectroscopy, time-histories of H2O, OH and HO2 were measured behind reflected shock waves. The rate constant of reaction CH3 + HO2 CH3O + OH was determined to be 6.8 x 10(12) cm(3) mol(-1) s(-1) with an uncertainty factor of 1.4. The rate constant of the competing CH3 + HO2 CH4 + O-2 reaction was determined to be 4.4 x 10(12) cm(3) mol(-1) s(-1), with an uncertainty factor of 2.1. In addition, the rate constants of two other major reactions of the reacting system, H2O2 (+M) -> 2OH (+M) and OH + CH4 CH3O + OH, were found to have excellent agreement with values recommended in literature. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.