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-dinitro1,3,5,7-tetraazabicyclo[3,3,1] nonane (DPT) for the first time. When the nitrolysis of DPT was slowed down, two inter-mediates 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.