The phase diagram and thermal decomposition for binary mixtures of cyclotrimethylenetrinitramine (RDX) and nitroguanidine (NQ) were investigated by DSC and using a microscope with a heat stage (MHS). The eutectic compositions of the RDX/NQ binary system were obained by constructing a special phase diagram for the relation of the apparent fusion heat with the composition. The DSC results were confirmed by the phase diagram of liquidus temperature vs. composition constructed from MHS data.