Two intermediates, 1,5-dinitroso-3,7-dinitro-1,3,5,7-tetraazacyclooctane (DNDS) and 1-nitroso-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane (MNX), were isolated and characterized in the synthesis of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) from the nitrolysis of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane (DPT) for the first time. When the nitrolysis of DPT was slowed down, two intermediates were detected with HPLC. It was proposed that electrophilic NO2 and NO from HNO3 and N2O4 might attack nitrogen atoms at positions 3 and 7 of DPT to form the cations of the intermediates, then nucleophilic H2O attacked the bridge carbon atoms of DPT to produce the intermediates, which were oxidized to form HMX.