Laser-initiated decomposition of single aerosol particles in the source region of a time-of-flight spectrometer, followed by vacuum ultraviolet laser ionization and mass analysis of the reaction products, reveals the identity of species formed in the early stages of condensed-phase decomposition in nitromethane and nitromethane-diethylarnine mixtures. In pure nitromethane, the initial stages of decomposition are characterized by unimolecular processes. At near-threshold energies, the cleavage of the C-N bond to form CH3 and NO2 dominates the dynamics, and at higher energies we find evidence of rearrangement to methylnitrite, followed by the formation of CH3O and NO. In nitromethane seeded with 0.1% diethylamine, an additional channel is observed which produces ionic intermediates. Detection of protonated diethylamine and the nitromethane aci-anion confirm that an acid-base reaction is involved in this case. We also detect the radical anion, NO2-, whose production is correlated with an increase in methyl radical yield compared to the results for pure nitromethane. This correlation shows that NO2 is directly implicated in nitromethane decomposition. Furthermore, deuterium labeling studies indicate a hydrogen atom transfer reaction between the nitromethane aci-anion and neutral nitromethane that produces the weakly bound nitromethane anion, the precursor to CH3 and NO2. These data support and extend a sensitization mechanism proposed by Gruzdkov and Gupta [Gruzdkov, Y. A.; Gupta, Y. M. J. Phys. Chem. A 1998, 102, 2322] and discriminate against others.