Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) undergoes almost a simultaneous transition from melting to decomposition, which makes direct measurement of the heat of fusion from calorimetric studies very difficult. The heat of fusion of HMX reported in the literature is unusually high when compared to those of energetic materials with similar molecular structures such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and [3-nitrooxy-2,2-bis(nitrooxymethyl)propyl] nitrate. In this work, we investigate melting properties with differential scanning calorimetry (DSC) and theories of solubility thermodynamics. A series of DSC experiments with heating rates 0.2520 K/min show that melting onset of HMX starts appearing at heating rates between 5.56.0 K/min. Since the melting endotherm is convoluted with the decomposition exotherm, we determine the heat of fusion of HMX based on solubility thermodynamics by measuring the melting point depression of HMX in the presence of RDX. The heat of fusion obtained from the experiments is 31.9 +/- 3.9 kJ/mol. The measured heat of fusion of HMX is further collaborated with the value calculated from a solubility thermodynamics modeling of HMX in various solvents using the nonrandom two liquid segment activity coefficient model, which is 31.2 +/- 9.4 kJ/mol.