Previous studies have suggested that a high-pressure form of the widely used secondary explosive RDX can be obtained above ~4 GPa and 488 K. This phase is reported to be sufficiently metastable that it can be recovered almost to ambient pressure and temperature. This proposal seeks to identify and structurally characterise this elusive form using the PEARL-HiPr instrument and a recently developed variable temperature device. An equation of state for this phase will also be obtained, and its stability to both pressure and temperature will be explored. This information will be of very great value to those involved in the modelling of the properties and performance of RDX under operational conditions, and will be of wider significance for the testing of ab initio crystal-structure crystal-structure predictions and molecular dynamics simulations.