Tautomerization, ionization, and bond dissociations of the insensitive high-energy explosive 5-nitro-2,4-dihydro-3H- 1,2,4-triazolone (NTO) were studied by molecular orbital SCF and MP2 theories and with the Becke3LYP hybrid density functional using the 6-31+G* and 6-311+G** basis sets. Energies computed with these methods were compared against accurate G2 energies for the tautomerization, ionization, and bond dissociations of nitromethane. The Becke3LYP and MP2/6-31+G** structures of NTO anion 9 compare well with the reported X-ray structure of the NTO diaminoguaniainium salt. The TR frequencies calculated with Becke3LYP compare well with those observed for crystalline NTO. Two enol tautomers (2 and 4) with 1H-1,2,4-triazole skeletons are only 4 kcal/mol at MP2/6-311+G** (and 9 kcal/mol at Becke3LYP/6-311+G**) less stable than NTO, while the two nci-nitro tautomers (5 and 6) are ca, 30 kcal/mol less stable than NTO. Comparisons of bond lengths. calculated proton chemical shifts, and magnetic susceptibility anisotropies show the enol tautomers are more aromatic than NTO, which accounts for their enhanced stabilities. NTO anion 9 is also more aromatic than NTO, which partly explains its small proton affinity of 321 kcal/mol calculated at Becke3LYP16-311+G**+ZPE. The estimated N-H and C-NO2 bond dissociation energies for NTO are respectively 93 and 70 kcal/mol, and that for the N-OH bond of the nci-nitro tautomer 5 is 35 kcal/mol. Some possible processes in the initial stages of NTO decomposition are bimolecular hydrogen atom transfers (in the condensed phase), C-NO2 bond homolysis (at high temperatures or under conditions of shock or impact), and homolysis of the N-OH bond in the aci-nitro tautomers.