The vacuum ultraviolet photolysis and mass spectral cracking pattern have been used to elucidate the chemistry occurring during the gas phase γ radiolysis of ethylenimine. In the 147 nm photolysis of ethylenimine, the major primary reactions occurring are (1) C2H4NH→C2H4+NH, φ=0.38, and (2) C2H4NH→CH3+[H2CN], φ=0.47. The mass spectrum of ethylenimine indicates that in the γ radiolysis of ethylenimine, where ions as well as neutral excited states are produced, the mechanism includes the ionic analog of Reaction (2): C2H4NH+→CH3+H2CN+. The mass spectrum also suggests the inclusion of Reaction (10) in the γ-radiolysis mechanism: (10) C2H4NH+→C2H4N+ + H. Although it cannot be proven conclusively, because hydrogen atoms and ethylenimine radicals are produced in secondary reactions, spectroscopic evidence suggests that the neutral analog of Reaction (10) may occur in the 147 nm photolysis: (9) C2H4NH→C2H4N+H. Except for reaction (1), which has no ionic analog in the mass spectrometer and which probably occurs from a valence excited state, the similarity of the neutral and ionic reactions occurring in the 147 nm photolysis and γ radiolysis of ethylenimine suggests that the Rydberg excited state, (nN→4p), populated by 147 nm photons exhibits quasi-ionic behavior. It seems likely that useful correlations of vacuum ultraviolet photolyses (8–12 eV), γ radiolyses, and mass spectra should be possible for many polyatomic molecules which frequently exhibit Rydberg excitations at these energies.