The effects of pressure on the thermal decomposition kinetics, chemical reactivity, and phase behavior of RDX have been studied by a combination of measurement techniques in conjunction with a high-pressure diamond anvil cell. These techniques include (1) Fourier transform infrared (FTIR) spectroscopy for kinetic measurements and phase identification, (2) energy dispersive x-ray powder diffraction for identification of the observed polymorphic forms and also compression measurements, and (3) optical polarizing microscopy for visual detection and confirmation of phase transformations and determinations of transition pressures. The ruby method of pressure measurement was used in all methods employed. Studies were generally limited to the region where decomposition rates could be measured within reasonable laboratory time, i.e., below 10 GPa and 573 K. The P-T phase diagram for RDX was determined to 573 K and 7.0 GPa, delineating the stability fields of three solid phases, ct, /3, and 7, and the liquidus. The c~ and/3 phases of RDX were found to thermally decompose, while the 3' phase transformed to either c~ or/~ before reaching decomposition temperatures. The decomposition rate of ct phase was found to increase with increasing pressure, suggesting a bimolecular-type mechanism. An activation energy of 51 kcal/mol and activation volume of -5.6 cm3/mol were calculated for the thermal decomposition reaction of a RDX.