Photochemical reactions of hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) are studied using FT-IR spectroscopy. Small single crystal and crystalline powder pelleted with KBr are irradiated with UV light at liquid nitrogen temperature, and the reaction progress is monitored by taking IR spectra. The same fragments, N2O, NO, CO2, and N2O4, are produced irrespective of the crystal size. The reaction products suggest that the N-N bond fission and the heterogeneous reactions such as the C-N bond breaking are major primary pathways and that the concerted depolymerization into CH2NNO2 is not effective. Physical states of the fragments produced in different sample conditions are discussed based on the IR band shapes.