Abstract <p>The formation of positive and negative ions in the homologous series of trinitrobenzene, trinitrotoluene, and trinitroxylene was studied by mass spectrometry. Positive and negative ion mass spectra were acquired through electron ionization and resonant electron capture methods on a quadrupole mass spectrometer within a GC–MS system. The ionic compositions of the opposite-sign spectra display both similarities and complementary differences that aid in compound identification. These features are interpreted through quantum chemical calculations of the thermal effects associated with molecular ion fragmentation. For trinitrobenzene, close correspondence between the composition of positive and negative ion peaks in the spectra reflects comparable energy requirements for nitro group elimination via the simple C–N bond cleavage and the nearly equal exothermicity of NO release through the nitro–nitrite rearrangement. In contrast, the methyl-substituted compounds, trinitrotoluene and trinitroxylene, exhibit pronounced differences: their positive ion spectra are dominated by products of multistage decays initiated by the exothermically favorable release of a hydroxyl group due to the ortho effect, whereas the analogous processes in negative ions demand additional energy input. The observed abundance of exothermic fragmentation pathways provides insight into the relative stability of molecular ions and suggests parallels with the mechanisms of explosive transformations.</p>