To improve the mechanistic understanding of the possible decomposition in the gas phase of the energetic material HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), we used ab initio calculations to determine the various unimolecular decomposition channels. We find three distinct mechanisms: (i) homolytic cleavage of N-N bond to form NO2 (M = 46) and HMR (M = 250) which subsequently decomposes to form various products (ii) successive HONO eliminations to give four HONO (M = 47) plus a stable intermediate (M 108); (iii) O-migration from one of the NO2 groups of HMX to neighboring C atom followed by the decomposition of intermediate (M = 296) to INT222 (a ring-opened RDX structure) and MN-oring (M = 74), which can undergo dissociation to smaller mass fragments. The decomposition scheme for HMX is similar to that for RDX presented earlier (J. Phys. Chem. A 2000, 104, 2261), except that concerted decomposition of HMX to four MN (M = 74) molecules is not a favorable decomposition pathway, whereas this pathway was found in RDX decomposition (both experimentally and theoretically). The formation of RDR-o in the N-N homolysis pathway 1 or the formation of INT222 in pathways 1 and 3 presents an unified mechanistic scheme for the decomposition of both of these nitramines. The HMX decomposition mechanism correlates with available condensed phase experimental results, but detailed comparison of the predicted gas phase energetics is not possible.