The solid state thermochemical decomposition kinetics and activation energy of neat 1,3,5,5-tetranitrohexahydropyrimidine (DNNC) and its DNNC-d(6) deuterium labeled analogue were obtained by isothermal differential scanning calorimetry (IDSC) at 142, 145, and 148 C. Global rate constants and kinetic deuterium isotope effect (KDIE) data from the exothermic decomposition process suggest that homolytic C-H bond rupture, in one or both types of chemically non-equivalent methylene (-CH2) groups of the DNNC ring structure. constitutes the exothermic rate-controlling step. A DNNC-d(6) energy of activation equal to 115 kJ/mol was determined for this initial autocatalytic exothermic energy release from which a 106 kJ/mol activation energy was calculated for unlabeled DNNC. This exothermic autocatalytic decomposition process follows an extended endothermic induction period for DNNC which shows a higher 128 kJ/mol activation energy during which a catalytic initiating species may form by a rate-controlling step different from C-H bond rupture. (C) 2005 Elsevier B.V. All rights reserved.