The kinetics and mechanism of the thermal decomposition of 3-nitro-1,2,4-triazole-5-one (NTO), a heterocyclic nitro triazole, have been studied, together with its morphology and evolved gaseous products, using thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy (IR), differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and hot-stage microscopy. The crystal structure parameters obtained from the XRD pattern are a = 20.000 +/- 0.005 Angstrom, b = 20.000 +/- 0.005 Angstrom, and c = 6.573 +/- 0.005 Angstrom. The crystal belongs to the tetragonal system, with a c/a ratio of 0.329. The IR spectrum of NTO has also been recorded and the bands assigned. The kinetics of the thermolysis has been followed by both isothermal TG and IR. The best linearity, with a correlation coefficient of 0.9937, was obtained for the Avrami-Erofe’ev equation, it = 3, in the range 0-83% alpha, using isothermal TG. The activation energy was found to be 186 kJ mol(-1), with log(A/s(-1)) being 16.64. The effect of a series of additives, incorporated to an extent of 5%, on the initial thermolysis of NTO has also been studied. Evolved gas analysis by IR showed that CO2, NO2, NO and N2O are produced in larger amounts than CO and HCN. The cleavage of the C-N bond appears to be the primary step in the thermolysis of NTO.