Nitrobenzene was oxidized in supercritical,water in a plug-flow reactor. The experimental conditions included a temperature range from 378 to 600 degreesC, a pressure range from 250 to 346 atm, and residence times between 80 and 180 s. The disappearance of nitrobenzene during supercritical water oxidation exhibited a global rate law that was 1.04 +/- 0.11 order in nitrobenzene, 0.49 +/- 0.16 in oxygen, and 0.07 +/-0.10 in water. The activation energy was 36 000 +/- 5000 J mol(-1), and the Arrhenius preexponential factor was 34.9 +/-10.2 s(-1). When nitrobenzene decomposed, greater than 90% of the organic nitrogen was converted to inorganic species, mainly elemental nitrogen, and greater than 60% of the organic carbon was transformed into carbon monoxide (CO) and carbon dioxide (CO2) with a high selectivity toward CO2. Nearly complete (>95%) mineralization was achieved at T = 600 degreesC, P = 346 atm, and a residence time of 160s.