The thermal decomposition of two highly energetic materials, mononitrobiuret (MNB) and 1,5-dinitrobiuret (DNB), was investigated by thermogravimetric analysis and differential scanning calorimetry. Mass spectrometry and IR spectroscopy were used to identify the gaseous products. Decomposition appears in both cases to be initiated by the release of nitramine. The gaseous products after the exothermic decomposition are similar for MNB and DNB, but the decomposition temperature of MNB is significantly higher and leads to the formation of urea, biuret (imidodicarbonic diamide), triuret (diimidotricarbonic diamide), tetrauret (triimidotetracarbonic diamide), and cyanuric acid (S-triazine-2,4,6(1H,3H,4H)-trione).