The thermal decomposition of ammonium permanganate was studied by thermogravimetry-mass spectrometry (TG-MS). A Perkin Elmer TGS-2 thermobalance and a Hiden's HAL 3F/PIC mass spectrometer equipped with a fast ion counter was used. Products of thermal decomposition of ammonium permanganate are oxygen, manganese oxides, water and ammonia. There is no gas phase oxidation of ammonia by oxygen. The possible intermediates of low-temperature decomposition reaction are ammonium nitrite or nitrate which transform into N-2, N2O, or H2O. The oxidation of adsorbed ammonia by manganese oxides via the superoxide active centers takes place on the surface of manganese oxide.