The first kinetic measurements for aqueous [NH3OH]NO3 that are based on species concentrations at hydrothermal conditions are discussed. The decomposition sate of 0.1 and 0.2 m [NH3OH]NO3 was determined in real time by flow-cell LR spectroscopy at 463-523 K and 27.5 MPa. The loss of NH3OH+ (2730 cm(-1) bending combination) and formation of N2O (nu(3) at 2230 cm(-1)) occur at the same rate under these conditions and were fit to a first-order rate expression. Using all of the data, E-a 103 +/-21 kJ/mol and ln(A/s) = 21 +/- 5. The decomposition rate resembles that of solid [NH3OH]NO3 in a similar temperature range, but the kinetics for dilute solutions and the solid state are too slow to apply to the combustion rate of a more concentrated solution of [NH3OH]NO3.