The thermal decomposition and thermal stability of 1,3,5-trinitro- 2-oxo-1,3,5-triazacyclohexane (keto-RDX or K-6) was studied. The keto-RDX synthesis is described, mass spectra (electron impact (70 eV) and chemical ionization) similar to RDX spectra registered under identical conditions are presented, and mass spectroscopy fragmentation paths are proposed. The LI-MS (laser induced/mass spectroscopic) results imply that the first step in the decomposition of keto-RDX is the elimination of NO2 or HONO and subsequent breakdown of the triazacyclohexane ring. The thermal stability, activation energy (E(a) = 140 kJ mol(-1)), and frequency factor (K-0 = 9 x 10(9) s(-1)) in the temperature interval 90-120 degrees C were measured using chemiluminescence (NO detection only). The activation energy was also determined from DSC data using the ASTM method E 698-79, and was found to be 280 kJ mol(-1) with a frequency factor of 7.0 x 10(29) s(-1) in the temperature interval 175-200 degrees C. Microcalorimetry, drop-weight test, friction test, and ignition temperature (Wood’s metal hath) measurements were also conducted. Quantum mechanical calculations (semi-empirical MNDO method with PM3 set at the unrestricted Hartree-Fock level) were conducted to correlate the sensitivity and thermal decomposition with those of RDX. No significant differences in bond-breaking energies for RDX and keto-RDX were found. Conclusions drawn from the experiments are that the decomposition of keto-RDX is auto-catalytic, and that the sensitivity of keto-RDX is not connected with the initial bond-breaking step. More than one method for measuring the risk involved in handling an explosive is necessary since the sensitivity depends on different stages in the decomposition.