This paper presents experimental measurements of 50 mass% hydroxylamine (HA)/water thermal decomposition in air and vacuum environments using an automatic pressure tracking adiabatic calorimeter (APTAC). Overall kinetics, onset temperatures, non-condensable pressures, times to maximum rate, heat and pressure rates versus temperature, and mixture vapor pressures for the experiments in vacuum were similar when compared to the corresponding data for HA decomposition in air. Determined was an overall activation energy of 119 +/- 8 kJ/mol (29 +/-2 kcal/mol), which is low compared to 257 kJ/mol (61.3 kcal/mol) required to break the H2N-OH bond reported in the literature. The availability of oxygen from air did not affect detected runaway decomposition products, which were H-2, N-2, N2O, NO, and NH3, for samples fun in vacuum or with air above the sample. A DeltaH(rxn) of -117 kJ/mol (28 kcal/mol) was estimated for the HA decomposition reaction under runaway conditions. (C) 2002 Elsevier Science B.V. All rights reserved.