Raman spectroscopy was used to determine the vibrational structure and the stability of the high-pressure-high-temperature (HP-HT) polymorph of RDX after it had been quenched to room temperature. Although this polymorph has limited chemical stability under high pressure and temperature, we show that it is chemically and structurally stable from 0.6 GPa to at least 20 GPa at room temperature. Below 0.6 GPa, it readily converts to the alpha-polymorph. Pressure dependence of the vibrational structure of the HP-HT polymorph was measured and compared with the vibrational structures of other known RDX polymorphs: alpha, beta, and gamma. In contrast with previous suggestions, our data indicate that the HP-HT polymorph can have a different structure than the beta-polymorph. This finding supports the recent suggestion that the HP-HT polymorph should be given a separate designation, epsilon-RDX. Furthermore, symmetry correlation analyses of Raman spectra indicate that the-HP-HT polymorph (epsilon-RDX) may assume the space group isomorphous with the C-2v[C-1(4)] point group and with molecules adopting the pseudo-AAA conformation.