Isotopically labeled 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) was used to determine the origins of nitrogen atoms in its decomposition gases. NTO samples with N-15 labels at 1 and 2 positions, the 4 position, and the 6 position of NTO were synthesized.(1) The thermolysis of NTO yields a brown insoluble residue and the nitrogen-containing gases dinitrogen, nitrous oxide, nitric oxide, and hydrogen cyanide. The analysis of these nitrogen-containing gases is reported, and the mechanistic implications are considered.