The phase diagram of HMX/DMSO and the solvated complex HMX-(DMSO)(2) has been investigated by visual observation, high performance liquid chromatography (HPLC), differential scanning calorimetry (DSC), and optical microscopy. Visual observations showed the solved complex exhibited an incongruent melting point, i.e., the melting point of the solvated crystal occurs within the region of the phase diagram where HMX itself precipitates. If the solvated crystal is removed from solution and heated, it reverts to solid HMX and DMSO at the peritectic temperature (similar to 70 degrees C). Using HPLC the change in concentration of the liquid phase was monitored as a function of temperature to confirm the phase line of the DMSO/solvated complex based on visual observation. Polarized light microscopy of the dissolution of seeded solutions of different concentrations was also used to verify these results. It is possible that a 1 : 1 complex also exists, but no evidence for such a complex was found in these experiments.