The concentrations of O, OH, and H 2O were measured during the reflected-shock initiated reaction of CH 4−O 2−Ar mixtures using various spectroscopic techniques. Concentration data were obtained for three different CH 4−O 2−Ar mixtures, having CH 4/O 2 ratios of 0.5, 1, and 2, for initial reflected-shock temperatures in the range 1875 o–2240 oK. Following an induction period, the O and OH concentrations attained maximum values which were significantly greater than the final equilibrium concentrations. During the initial stages of these concentration overshoots, the observed species concentrations were not consistent with an assumption of partial equilibration of the rapid bimolecular reactions of the H 2−O 2 chain. A proposed 23-reaction methane oxidation mechanism correctly predicts the time of occurrence and magnitude of the radical concentration overshoots, and also predicts a departure from a partial equilibrium state during the initial stages of the radical concentration overshoots. Rate constants for several elementary reactions in the proposed mechanism were inferred by a comparison of the observed and predicted concentration profiles. These reactions and the inferred rate constants are (R1) CH 4+M→CH 3+H+M, k=1.4×10 17 exp (−44 500/ T) cm 3/mole·sec (R7) H+O 2→O+OH, k=6×10 14 exp (−8450/ T) cm 3/mole·sec (R15) CH 3+O→H 2CO+H, k=1×10 14 cm 3/mole·sec (R16) CH 3+O 2→H 2CO+OH, k=1.2×10 11 exp (−5000/ T) cm 3/mole·sec (R25) H 2CO+M→HCO+H+M k 25=1×10 1 exp (−18500/ T) cm 3/mole·sec in the temperature range 1900 o–2400 oK.