Decomposition hazards of hydroxylamine (HA)/water solution with and without the addition of iron ion were studied in this paper. Tests were conducted to obtain information about decomposition hazards of HA/water solution following the United Nations recommendations on the transport of dangerous goods. When the heat accumulation storage test was conducted using HA50 wt.%/water solution without the addition of iron ion, the self-accelerating decomposition temperature (SADT) was 80 degreesC. Therefore, HA50 wt.%/water solution was not classified with self-reactive substances following the United Nations recommendations on the transport of dangerous goods. Decomposition hazards of HA50 wt.%/water solution with added iron ion were investigated. The concentration of iron ion in sample of HA50 wt.%/water solution without the addition of iron ion was below 1.0 ppm. The range of the concentration of iron ion in sample of HA50 wt.%/water solution with added iron ion was between 1.0 and 5.4 ppm. The thermal stability of HA50 wt.%/water solution decreased by the addition of iron ion in the heat accumulation storage test. HA50 wt.%/water solution with the addition of iron ion was classified with self-reactive substances following the United Nations recommendations on the transport of dangerous goods. The intensity of the thermal decomposition of HA50 wt.%/water solution increased by the addition of iron ion in the Koenen test. (C) 2003 Elsevier B.V. All rights reserved.