The pathways corresponding to the most energetically favorable decomposition reactions that can be envisaged for o-nitrotoluene (and 20 of its derivatives) have been studied, using density functional theory, in order to evaluate the influence of substituents' nature (nitro, methyl, amino, carboxylic acid, and hydroxyl) and position. The first mechanism consists of the direct dissociation (homolysis) of the carbon nitrogen bond (CH3C6H4NO2 = CH3C6H4 + NO2) whereas the second one is a more complex process initiated by C-H alpha attack and leading to the formation of anthranil and water (C6H4C(H)ON + H2O). For each compound, the initial step of this last channel is the rate limiting one, the Gibbs activation energy of all systems being very close, that is all in the 40-44 kcal/mol range. More important variations have been observed for the C-NO2 homolysis Gibbs activation energies (46-60 kcal/mol). These variations have been related to electron donor-acceptor properties of substituents by considering significant correlations (R-2 > 0.9) with the Hammett parameters (sigma). Nevertheless, though the influence of substituents on the direct breaking of the C-NO2 bond was important, the C-H alpha attack remained finally the major decomposition channel for the studied compounds. Our study underlines the complexity of the decomposition process in nitroaromatic compounds and casts some doubts on the characterization of the energetic properties of such molecules only on the basis of C-NO2 homolysis.