The reaction phase diagram of hexahydro-1,3,5-trinitro-s-triazine (RDX) has been studied as a function of temperature and pressure by Raman spectroscopy to 29 GPa and temperatures ranging from 4 to 298 K. Three stable phases (alpha, gamma, and delta) have been found and their phase stabilities have been investigated. Phase boundaries were studied as a function of pressure and temperature, permitting a delineation of the various polymorph stability fields. A pressure-temperature reaction/phase diagram is constructed from the results of this study and compared to previous high temperature work.