The unimolecular dissociation of RDX (hexahydro-1,3 5-trinitro-1,3,5-triazine) in liquid xenon is investigated to determine condensed-phase effects on the N-N bond fission and ring-opening reactions. The dependence of the rate constants on pressure at a fixed temperature is studied using molecular dynamics simulations, and the result is consistent with the experimental finding that the ring-opening channel is suppressed in a condensed-phase environment. The effects of intramolecular vibrational energy redistribution (IVR) and intermolecular energy transfer on reaction rates are also studied by putting a "hot" RDX molecule in liquid xenon. The reaction rates are calculated using a statistical approach and direct simulations. The statistical rate for the bond fission is 45% larger than the corresponding dynamical one, indicating that the rate of IVR is not faster than that of reaction.