One of the aims of this proposal is to quench to ambient pressure a metastable form of the widely used secondary explosive RDX. This is formed above ~4.5 GPa and 448 K but is sufficiently metastable that it can be recovered almost to ambient pressure and temperature, and has implications for the shock-sensitivity of munitions. Structural information obtained from 120 K to the transition pressure should provide insight into the mechanism of the transition and may have implications for observed variation in the shock sensitivity of commercial RDX. Experiments on HMX will clarify the previously reported pressure- and temperature-induced phase transitions and may identify new polymorphs of HMX. These transitions have important implications for the modelling of the performance characteristics of HMX and may play a role in the observed pressure dependence of the burning rate of HMX.