The risk evaluation of decomposition of hydroxylamine(HA)/water solution was studied experimentally. The thermal property of HA/water solution was studied from the calorimetric data obtained using the differential thermal analysis (DTA). The intensity of decomposition was studied on the basis of the results of the mini closed pressure vessel test (MCPVT) and the pressure vessel test (PVT) in addition to the steel tube test. The thermal property of HA/water solution was evaluated on the basis of results of the DTA. The heat-release onset temperatures using the no-treated stainless steel cells were more than 70 K below those measured using the GSC. This result implies that the heat-release onset temperature depends on the materials of sample cell. On the other hand, the heat of reaction did not depend on the materials of sample cell. The intensity of the thermal decomposition was investigated on the basis of results of the MCPVT, the PVT and the steel tube test. The intensity of the thermal decomposition increased as the HA concentration increased in the MCPVT. The intensity of the thermal decomposition increased greatly when the HA concentration was beyond 80wt.% in the MCPVT. It was elucidated that the thermal decomposition of HA 70wt.%/water solutions was very violent in the PVT. In addition, HA/water solutions of more than 80wt.% concentration could detonate in the steel tube test. HA 80 wt.% water/solution was easily detonated by a detonator without RDX in the steel tube test. In addition, the decomposition hazard of HA/water solution by the metal ion and the iron powder was studied in this paper. The thermal stability of HA85%/water solution with the iron ion or the iron powder was discussed on the basis of the heat-release onset temperature by the DTA. The heat-release onset temperatures decreased when the concentration of the iron ion or the iron powder increased in the DTA measurements. The reactiveness of HA/water solution with the metal ion of iron, manganese, nickel, chromium and copper was examined by measuring the mass loss of HA/water solution after the metal ion was added to HA/water solution at room temperature. The reactiveness of HA/water solution with the iron powder was also studied in this paper. The ferrous ion, the ferric ion and the iron powder reacted with HA/water solution. Ignition automatically began when the 0.2wt.% ferric ion solution was added to HA85wt.%/water solution. The mass loss rate depended on the HA concentration greater than the iron ion concentration. The mass loss rate increased when an amount of the iron powder increased. On the other hand, the decomposition reaction of HA85wt.%/water solution with Cu2+ was calm compared to that of the iron ion. HA/water solution did not react with Mn2+, Ni2+ and Cr3+ at room temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.