The bimolecular rate constants of the addition reaction between hydroxyl radical ((OH)-O-.) and nitrobenzene (C6H5NO2) were measured in subcritical and supercritical water (SCW) at temperatures between ambient and 390degreesC. The measured bimolecular rate constants showed distinctly non-Arrhenius behavior (i.e., essentially no increase with temperature) from ambient to 350 degreesC, but increased in the slightly subcritical and supercritical region between 350 and 390 degreesC. These data were modeled reasonably well over the entire temperature range with a three-step reaction mechanism, originally proposed by Ashton et al.(1) This model includes the formation of a pi-complex intermediate as the precursor of the nitrohydroxycyclohexadienyl radical.