Monomethylammonium nitrate (MMAN) was studied because it is used as a sensitizer in commercial water-gel explosives. Because of its extremely hygroscopic nature, MMAN crystals were placed in open pans in the DSC and temperature-cycled to eliminate water, so as to obtain accurate values for the solid-solid transition and for the melting point. The exothermic behaviour of MMAN was studied by isothermal and non-isothermal DSC in hermetic pans, by TGA, and also by accelerating rate calorimetry (ARC(TM)). The results were compared to those for ammonium nitrate and the limited data on MMAN available in the literature. Extrapolation of DSC data on the onset temperature of the exotherm yields reasonable agreement with the large-scale ARC(TM) tests, providing evidence that the onset temperature can be considered as a practical parameter for hazard analysis of energetic materials.