The thermal decomposition of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) was examined over the temperature range 220-280 degrees C using both neat NTO and NTO in aqueous and methanol solutions. The decomposition of NTO in the presence of 2,4,6-trinitrotoluene (TNT) was also examined. Activation parameters were calculated for all thermolyses. When mixtures of NTO and TNT were decomposed, the decomposition rate of both components was enhanced. Additives such as NO2, ammonia, nitric acid, and ammonium nitrate also accelerated the decomposition of NTO and of TNT. Decomposition of deuterated NTO neat and in solution was noticeably slower than that of proteo-NTO, indicative of an intramolecular deuterium kinetic isotope effect (DKIE). An intermolecular DKIE was-also observed when NTO was decomposed in deuterated solvent or in a mixture with TNT-d(3). It was concluded that the rate-determining step in at least one NTO thermolysis pathway involved hydrogen transfer to the nitro group followed by subsequent:loss of HONO. This mechanism is believed to be dominant at low temperatures, while at high temperatures C-NO2 homolysis is probably a competitive decomposition pathway. Decomposition products from NTO thermolysis, neat, in solution, and in admixture with TNT, are reported for the first time. The condensed-phase thermolyses produced small quantities of 2,4-dihydro-3H-1,2,4-triazol-3-one (TO) and large amounts of an intractable tar for which the elemental ratio was roughly that of TO. Because TO was observed as a NTO decomposition product, its thermal decomposition behavior was examined in the same fashion as that of NTO. LikeNTO/TNT mixtures, TO/TNT mixtures exhibited mutual destabilization. However, unlike NTO, TO exhibited neither an internor ari intramolecular DKIE, and its decomposition rate was unaffected by the presence of NO2. The elemental analysis of the TO residue differed from that of thermolyzed NTO. It was concluded that TO, while formed in the decomposition of NTO, was not an intermediate in the principal NTO decomposition pathway. Furthermore, while it appeared that NTO and TO promoted TNT decom;position by the same mechanism, hydrogen transfer to an arerie nitro group, the mode by which TNT enhanced the decomposition of the two triazolone rings, differed. The kinetics and products of decomposition of eight other triazole-like rings were examined. Thermal stability was related to ring substitution; rings with NO2 and NH2 substituents decomposed more rapidly at 270 degrees C than those with O or H on carbon. The enhancement of decomposition under NO2 atmosphere was observed only on rings substituted with NH2 or O groups, suggesting the carbonyl of NTO is the point subject to NO2 attack. Hydrogen transfer to the nitro of NTO was supported by the observation that only rings substituted with either NO2 or NH2 exhibited an intermolecular DKIE.