The rate of reaction of nitromethane in the gas phase is little changed by the presence of O-2 or NO. The major products are CO, NO and H2O, plus NO2 if O-2 is included. The corresponding catalytic reaction over fresh Co-ZSM5 commences at a much lower temperature and gives primarily CO2 and NH3, the latter being oxidised to N-2 above 340 degrees C. As the catalyst ages below 320 degrees C HNCO eventually becomes the major nitrogen-containing product although it can be hydrolysed by added water. The results demonstrate one possible route for the methane-SCR reaction.