Initial steps for several proposed decomposition mechanisms of NTO have been studied theoretically in order to determine the energies of various reaction intermediates. The methods applied were restricted Hartree-Fock self-consistent field (SCF), single-and double-excitation configuration interaction (CISD), CISD corrected for unlinked quadruple excitations (CISD+Q), and coupled cluster including all single and double substitutions (CCSD) with a double-xi plus polarization (DZP) basis set. Harmonic vibrational frequencies were computed to characterize the species at the stationary points. The previously reported mechanisms examined include decomposition initiated by C-NO2 homolysis, migration of ring substituents, and ring-opening processes. On the basis of the energetics of the various schemes, our computations suggest that C-NO2 bond homolysis is the most probable initial step for unimolecular decomposition of NTO.