AbstractThe mass spectrum of nitromethane points to rupture of the CH3NO2 bond as the dominant primary reaction, as also observed in pyrolysis, photolysis and radiolysis. Isomerization of the molecular ion to the nitrite configuration seems to contribute little in the mass spectrum of nitromethane, in contrast to those of nitrobenzene and other nitroarenes. The nitrite ion is probably the immediate precursor of [NO]+ at its appearance potential, but most of the [NO]+ yield seems to stem from secondary decomposition of excited [NO2]+.