The thermal decomposition of ammonium nitrate (AN), activated carbon (AC), and potassium chloride (KCl) mixtures are investigated. The thermal properties were studied using differential scanning calorimetry (DSC) and the evolved gas was analyzed using thermogravimetry with mass spectroscopy (TG-DTA-MS). The time to maximum rate (TMRad) was then calculated using commercial thermal analysis software. DSC measurements of an AN/KCl mixture in a sealed sample pan showed a sharp exothermic decomposition and lower onset temperature than pure AN, whereas AN/KCl in an open pan exhibited an endothermic reaction. A mixture of AN/AC/KCl exhibited a lower onset temperature than both AN/AC and AN/KCl. TG-DTA-MS results revealed HCl gas was evolved from AN/KCl, which indicated the reaction of KCl with HNO₃ dissociated from AN to form HCl, and the subsequent destabilization of AN and AN/AC by HCl. TMRad calculations showed that AN/KCl underwent a run-away reaction within one day under closed adiabatic conditions above 180°C.