Three aspects of the catalytic reduction of NOx have been investigated with particular emphasis on Cu-ZSM5. The reaction using methane shows a large kinetic isotope effect which is absent with isobutane. Thus, while abstraction of hydrogen from methane is rate determining, a different step plays that role with higher hydrocarbons. With Cu-ZSM5 and Co-ZSM5 the equilibration step, NO to NO2, is incomplete below 400 degrees C in the presence and absence of methane. However, conversion of NO2 to NO is near complete when using isobutane over Cu-ZSM5, possibly due to a process which also maintains the catalyst free of deposited material. Measurements of the further reactions of isocyanic acid, HNCO, over Cu-ZSM5 at 270 degrees C show that its rate of hydrolysis to CO2 and NH3 is fast while that with NO to give N-2 and N2O is absent.