The unimolecular fragmentation of deuterium and oxygen O-18 isotopomers of the methyl nitrite radical cation (CH3ONO.+) was examined by means of tandem mass spectrometry. The [D-1] and [D-2] isotopomers were found to exhibit enormous intramolecular kinetic isotope effects associated with unimolecular H-./D-. losses. The dissociation rates and breakdown diagrams of energy-selected CH3ONO.+ and CD3ONO.+ ions were measured by photoelectron photoion coincidences. These data exhibit a modest isotope effect. The experimental results imply that the unimolecular H-. and NO. eliminations occur via a common intermediate, for which we suggest the distonic ion CH2O(H)NO.+. A new ionization energy of 10.44 eV for methyl nitrite is derived. Based on the experimental findings and the results of exploratory ab initio MO calculations, two models are presented for the unimolecular dissociation of the methyl nitrite radical cation, in which the initial step involves a 1,2-hydrogen atom migration, CH3ONO.+ --> CH2O(H)NO.+.