The effect of temperature, from ambient to supercritical, on the products of the reaction in water of HO. with benzene and the anions of pentabromophenol, pentachlorophenol, and pentafluorophenol is reported. Increasing temperature favored one-electron oxidation of the perbrominated and perchlorinated phenolate anions over adduct formation, yielding the corresponding phenoxyl radicals. In contrast, with benzene and pentafluorophenolate, the corresponding hydroxyl radical adduct was formed. The bimolecular rate constant for the self-reaction of the hydroxycyclohexadienyl radical was measured and its temperature dependence determined : for benzene, a kinetic analysis gave E-a of 48.0 kJ/mol and a preexponential term of 7.42 x 10(14) M-1 s(-1). The temperature dependence for the rate constant of the decay of the pentabromophenoxyl radical was non-Arrhenius, with a significant change in mechanism taking place at approximately 200 degrees C.