The thermal decomposition of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) has been studied by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Activation energies as a function of the extent of conversion, alpha, have been determined by model-free isoconversional analysis of these data. In open pans, evaporation is a prevalent process with an activation energy of similar to 100 kJ mol(-1). Confining the system in either a pierced pan or a closed pan promotes liquid state decomposition of RDX that occurs with an activation energy of similar to 200 kJ mol(-1), which suggests scission of an N-N bond as the primary decomposition step. In such a confined environment, gas phase decomposition is a competing channel with an activation energy estimated to be similar to 140 kJ mol(-1). In a closed pan, RDX generates a heat release of similar to 500 kJ mol(-1) that is independent of both the heating rate, beta, and the mass.