Ignition delay times have been measured behind reflected shock waves for combustible mixtures of methane seeded with NO2 or N2O. Two types of mixtures were investigated: mixtures diluted in Ar (97.15-97.5% dilution) to isolate the chemistry from gas dynamic effects and air/fuel mixtures to represent realistic conditions. Wide ranges of equivalence ratios (0.5, 1.0, and 2.0) and pressures (1-28 atm) were investigated. For the dilute cases, the baseline mixture was CH4/O-2/Ar (97.5% dilution), and NO2 or N2O was added to the baseline mixtures as 16.6% and 70.8% of the methane concentration by volume at (empty set) = 0.5, and at 16.6% of the methane concentration for the other equivalence ratios. For the fuel-air mixtures, a fixed NO2 addition (0.151% of the full mixture) was investigated. In general, the addition of NO2 and, to a lesser extent, N2O, reduces the ignition delay time. The reduction in the ignition delay time varies with the equivalence ratio and the pressure. Recent detailed kinetics mechanisms from the literature and a mechanism assembled from literature sub-mechanisms have been tested against these new data and against data from the literature. Overall, the mechanism proposed in this study presents good predictions of the data with NO2, well within about 20% over the entire range of conditions, while the predictions with N2O addition had similar accuracy. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.