The nitramines 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), and 1,3,5-trinitro-1,3,5-triazinane (RDX) are energetic materials commonly used in solid propellants and explosives. In order to predict ignition and deflagration of propellants containing these ingredients, their thermal decomposition behaviors must be thoroughly understood. In this study, the thermal decomposition of HMX was investigated using synergetic application of experimental and computational methods. Mole fraction profiles of the gaseous decomposition products evolving from the liquid-phase HMX were obtained using Fourier transform infrared (FTIR) spectroscopy for two types of thermolysis experiments - thermogravimetric analysis (TGA) and confined rapid thermolysis (CRT). Four heating rates (5, 10, 15, and 20 K/min) in TGA experiments and four set temperatures (290, 300, 310 and 320 degrees C) in CRT experiments were considered. In the TGA and differential scanning calorimetry (DSC) results, steep mass loss and rapid decomposition were observed after the melting of the HMX at 280 degrees C. CH2O and N2O were identified as the major decomposition products. Smaller quantities of H2O, HCN, NO and NO2, CO and CO2 were also formed. In the complementary computational study, liquid-phase elementary reactions were investigated using quantum mechanics calculations at B3LYP/6-311++G(d,p) level of theory with the conductor-like polarizable continuum model (CPCM). A zero-dimensional model was developed to simulate the TGA and CRT experiments based on conservation of mass and species in the condensed-phase and the gas-phase control volumes. The predicted mass loss and gas-phase mole fraction profiles of the decomposition products are in good agreements with the corresponding experimental results, indicating that the comprehensive mechanism proposed here captures the important reactions occurring during liquid-phase decomposition of HMX. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.