The oxidation peak potentials (E-p) of 31 phenolate, chlorophenolate, and other para-substituted phenolate ions were measured as a function of scan rate,substrate concentration, and pH by cyclic voltammetry in aqueous solution. The one-electron reduction potentials (E-red(o)) of the phenoxyl radical/phenolate ion couples were evaluated from E-p and the rate constants for dimerization of the phenoxyl radicals. The dependence of E-red(o) on pH was established, and the relationships that connect E-red(o) with the Brown sigma(+) constant, the pK(a) values of the phenols and the protonated phenoxyl radical cations, and the reduction potentials of the protonated radicals were explored.