Thermal decomposition of ammonium dinitramide (ADN) has been studied by differential scanning calorimetry (DSC) and thermal gravimetry combined with mass spectrometry (TO-MS). ADN decomposes in the range 130-230 degrees C with an overall heat release of 240 +/- 40 kJ mol(-1). The product gases have been identified by MS as NH3, H2O, NO, N2O, NO2, HONO, and HNO3. The global activation energy determined from TG experiments decreases from 175 +/- 20 kJ mol(-1) at the beginning of the reaction to 125 +/- 20 kJ mol(-1) when the reaction is near completion. Experiments conducted at different carrier gas flow rates indicate that the reaction is catalyzed by product gases released during the reaction. A condensed phase ionic reaction mechanism is proposed, in which ammonium nitrate and ammonium mononitramide (NH4NNO2) are formed as intermediates in two competing channels.