The objective of this study is to obtain information on the calorimetric behaviors of aqueous solutions of hydroxylamine (HA), hydroxylamine chloride (HACl), and hydroxylamine nitrate(HAN) caused by different Fe(III) states (free Fe(III) from Fe(NH4)(SO4)(2), Fe(CN)(6)(3-), and Fe(EDTA)(-)). The calorimetric data were obtained with a small-scaled reaction calorimeter, Super-CRC. In the mixing with Fe(III), HA showed the highest reactivity among three substrates. Free Fe(III) and Fe(EDTA)- showed catalytic effects in the reactions. In the overall heat of reactions, Fe(EDTA)(-) exceeded free Fe(III), which precipitated as Fe(OH)(3) and decreased the chances of interactions with HA. It was suggested that the generation of NH3 had taken place in the process of reducing HA along with Fe(II) oxidation. Fe(CN)(6)(3-) was less reactive than free Fe(III) and Fe(EDTA)(-). The ability of masking Fe(III) was estimated for CyDTA. The HA including CyDTA had no exothermic peak; however, there was an endothermic peak of the heat flow at Fe(III) injection. CyDTA was found to have the ability to inhibit a violent exothermic reaction of HA. (C) 2003 Elsevier Ltd. All rights reserved.