The supercritical water oxidation of ammonia was investigated in two isothermal, isobaric plug-flow reactors constructed of stainless steel and having different surface-to-volume ratios, at temperatures ranging from 803 to 903 K and pressures ranging from 14 to 28 MPa for 1-min-scale residence time under oxygen addition of 53-26% of the main reaction stoichiometry. Ammonia was mainly converted to nitrogen, while low levels (<1.6%) of nitrous oxide and nitrate ions were also produced. Ammonia was oxidized through both homogeneous and heterogeneous mechanisms, with the heterogeneous reaction being predominant. Pressure had an important effect in the homogeneous phase, although it could be concealed by the catalytic effect. of the wall when the surface-to-volume ratio was high. The reaction rates were calculated as 10(5.74) exp(-144.74/RT)[NH3](0.74)[H2O](-1.93) for the homogeneous reaction and 10(7.74)exp(-166/RT)[NH3](0.89) for the heterogeneous phase (activation energies are in kJ.mol(-1)).