Nanoscale crystals of cyclotrimethylenetrinitramine (RDX) were produced by Rapid Expansion of Supercritical Solutions (RESS). The experiments were performed by expansion of supercritical solutions of RDX in carbon dioxide through sapphire nozzles (ID: 100 and 150 mu m) at pressures of 15.0-29.5 MPa and temperatures of 343 - 348 K. Recrystallized particles were characterized by Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction, HPLC and melting point analysis. The process produced particles with a mean size in the range 110-220nm and a narrow size distribution. The product was crystalline as determined by X-ray diffraction. The effect of process conditions (T, P, nozzle diameter) on the crystal size distribution was determined.