The pyrolysis and oxidation of nitromethane (NM) were studied in a plug flow reactor over the temperature range of 735-1476K at 5Torr and in a jet-stirred reactor over the temperature range of 600-875 K at atmospheric pressure, respectively. Mole fraction profiles of major products and intermediates were identified with tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry. The current study represents the first comprehensive characterization of NM conversion under these conditions. The experimental results were compared to predictions with a detailed chemical kinetic model involving 364 species and 2389 reactions. The rate constants for significant dissociation reactions CH3NO2= CH3 + NO2 and CH3NO2 = HCNO+ H2O were calculated by the RRKM/Master Equation method using Variflex program in this work. The model provided an overall reasonable agreement with the measured data, but for pyrolysis conditions future work is required to improve predictions of some intermediates. Rate-of-production analysis indicates that the primary decomposition pathways of NM are different for pyrolysis and oxidation due to the effects of C-N bond fission and roaming mediated isomerization. From the sensitivity analysis, the two pathways mentioned above have promoting effects on NM consumption for all the studied conditions, while the reaction of CH3 +NO2 =CH3O+NO with inhibiting effect at Phi=2.0 shows promoting effects for Phi=0.4 and pyrolysis. This work extends the experimental database and helps to improve the understanding of low temperature chemistry of NM. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.