Raman spectroscopy and optical imaging were used to determine the phase boundaries between various hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) polymorphs. Experiments were performed on single crystals at pressures up to 8.0 GPa and temperatures ranging from room temperature to 550 K. Several distinct pressure regions were found in the RDX response at elevated temperatures: (i) melting of alpha-RDX followed by decomposition, below 2.0 GPa, (ii) decomposition of alpha-RDX between 2.0 and 2.8 GPa, (iii) irreversible transformation of alpha- and gamma-RDX to epsilon-RDX between 2.8 and 6.0 GPa, and (iv) decomposition of gamma-RDX above 6.0 GPa. A triple point between the alpha-, gamma-, and epsilon-RDX was found at 3.7 GPa and 466 K. The alpha-gamma phase transition was confirmed to occur at the same pressure, similar to 3.7 GPa, regardless of temperature, in the range of 295-460 K. Furthermore, it was determined that epsilon-RDX (i) has limited chemical stability under the pressure and temperature conditions where it is produced and (ii) decomposes according to the autocatalytic rate law. The findings reported here have provided new information about the response of RDX crystals at high pressures and temperatures.