Shock tube experiments and extensive numerical simulations were used to provide information required to construct a detailed chemical mechanism for the decomposition of gaseous nitromethane diluted in argon. Measurements were made of the time evolution of the pressure and the NO adsorption at a fixed location in the shock tube, and mass and infrared spectroscopy were used to identify the products. Experiments were performed in the pressure range 0.3 to 2.5 atms and temperature range 700 to 2000 K using mixtures of 10 to 100 percent nitromethane in argon. A mechanism composed of elementary reactions describing nitro-methane decomposition in the range of the experiments was compiled, tested and reduced to a set of 62 most significant reactions.