Differential scanning calorimetry and modulated differential scanning calorimetry were used to understand the decomposition mechanism of a developmental gas generant formulation. A eutectic mixture was formed by two fuels in the formulation. Differential scanning calorimetry data was used to construct a phase diagram The low melting point of the eutectic mixture explains why the formulation undergoes exothermic decomposition at a lower temperature than expected. Accelerating rate calorimetry was used to determine the temperature at which exothermic decomposition of the formulation occurs. A significant decrease in exotherm onset temperature occurred when fuel from a new supplier was used. This was detected by the accelerating rate calorimeter and the fuel was rejected.