Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show that the heating of 5-nitro-2,4-dihydro-3H- 1,2,4-triazol-3-one (NTO) leads to competitive sublimation and condensed-phase exothermic decomposition, Model-free isoconversional analysis has determined activation energies (E,,.) for these processes as a function of the extent of conversion, a. Sublimation occurs most readily in an open pan; although more than simple sublimation was observed, a global activation energy of E-alpha = 130-140 kJ mol(-1) for sublimation was determined. Nonisothermal TGA and DSC traces run on pierced pan samples provide convincing evidence for competitive sublimation and condensed-phase decomposition of NTO. Confining NTO samples in a closed pan results in condensed-phase decomposition that leads to the formation of gaseous reaction products and shows autocatalytic behavior during the latter stages. Isoconversional analysis of DSC traces of closed pan samples yield activation energies for exothermic decomposition that increase from E-alpha = 273 kJ mol(-1) for alpha = 0.01 to a plateau of 333 kJ mol(-1) for 0.17 less than or equal to alpha less than or equal to 0.35 prior to decreasing to 184 kJ mol(-1) for alpha = 0.99. The decrease in E-alpha with alpha during the latter stages of decomposition agrees with previous reports of autocatalytic behavior.