Thin films of homogeneously mixed nitromethane (CH3NO2) and D3-nitromethane (CD3NO2) ices were exposed to energetic electrons and Lyman alpha photons. The isotopically mixed reaction products were probed in situ via reflectron time-of-flight mass spectrometry coupled with pulsed photoionization during temperature programmed desorption. This study provides compelling evidence on three high energy pathways, which have not been observed under collisionless conditions in the gas phase. These are decomposition of nitromethane (CH3NO2) via atomic oxygen loss to nitrosomethane (CH3NO), fragmentation of nitromethane (CH3NO2) and/or methylnitrite (CH3ONO) via carbene (CH2) loss, and carbon-hydrogen bond rupture processes in nitromethane (CH3NO2) and/or methylnitrite (CH3ONO). (C) 2015 Elsevier B.V. All rights reserved.