The acid dissociation constant of the hydroxyl radical in aqueous solution was determined by pulse radiolysis from the [OH-] dependence of the rate constant for reaction of the (OH)-O-./O.- radical with benzoate ions. The rate constant and the pK(a)((OH)-O-.) values were determined over the temperature range 284-343 K. At 298 K, pK(a298)((OH)-O-.) = 11.54 +/- 0.04 and Delta(ion)Gdegrees(298)((OH)-O-.) = (65.9 +/- 0.3) kJ mol(-1). From the temperature dependence of K-a((OH)-O-.), we calculate Delta(ion)Hdegrees(298)((OH)-O-.) = (24.85 +/- 0.5) kJ mol(-1) and Delta(ion)Sdegrees(298)((OH)-O-.) = (-139 +/- 2) J mol(-1) K-1. The activation energy for the reaction of (OH)-O-. with C6H5CO2_ was found to be (8.8 +/- 0.6) kJ mol(-1) and the Arrhenius preexponential factor (2.4 +/- 0.6) x 10(11) L mol(-1) s(-1).