Time-resolved Raman and transient absorption studies on hydroxyl ((OH)-O-.) radical reactions with aqueous 4-carboxyphenol (-O2C-PhOH), the substrate in p-hydroxybenzoate hydroxylase, have led to the first direct observation of the basic form of a dihydroxycyclohexadienyl ((OH)-O-.-adduct of phenolate anion) radical and hydration-induced intramolecular electron transfer in this species. The base-catalyzed phenoxyl radical formation in this model system has been quantitatively described in terms of pK(a) of the phenolic proton (9.05) of the (OH)-O-.-adduct and the rate of OH- elimination (2.9 x 10(6) s(-1)) from its basic form. A small fraction, 12(+2)%, of the phenoxyl radical is formed via water elimination from the (OH)-O-.-adduct at the ipso position of the hydroxyl group (rate > 10(7)s(-1)). About 3% (OH)-O-. addition is seen at the carboxylic ipso position in basic solutions, which produces the p-benzosemiquinone radical anion (rate similar to 10(6)s(-1)). This work provides spectroscopic and kinetic evidence of the early chemical steps in the phenoxyl radical formation by (OH)-O-. oxidation and establishes the precise relationship between the formation rate and pH. A relationship between the rates of OH- elimination from the (OH)-O-. adducts of phenolate anions and pK(a) of the corresponding phenols is given.