The influence of the addition of ammonia on the oxidation of methane was investigated both experimentally and numerically. Experiments were carried out at atmospheric pressure, using a fused silica jet-stirred reactor, and a recrystallized alumina tubular reactor designed on purpose to reach temperatures as high as ∼2000 K. A temperature range of 600-1200 K was investigated in the jet-stirred reactor at a residence time of 1.5 s, while experiments in the flow reactor were carried out between 1200 and 2000 K, for a fixed residence time of about 25 ms in the reactive zone. A methane/ammonia mixture, diluted in helium, was used in both reactors with equivalence ratios varied between 0.5 and 2 in the first reactor, while stoichiometric conditions were investigated in the second one. The measurements indicate that CH4 reactivity was promoted by NH3 addition below 1200 K, but not so much influenced above. These results were interpreted and explained using a comprehensive kinetic model, previously validated over a wider range of operating conditions. The mechanism allowed to shed light on the underlying causes of the anticipated methane reactivity at low temperature, and of the major role played by NOx in it. This effect was shown to become less significant at higher temperatures, where the reactivity is mainly governed by H-abstractions on both fuels.