The optical absorption spectra, pK(a), and one-electron reduction potential (E-1) of the phenoxyl radical of 4,4 ' -biphenol (HOArArOH; Ar=C6H4) in aqueous solution are measured by the pulse radiolysis technique. At pH 4.8 the:absorption spectrum of the HOArArO radical shows two peaks at 385 nm (is an element of = 2223 m(2) mol(-1)) and 620 nm (is an element of = 867 m(2) mol(-1)). At pH 12.0, the respective peaks due to the -OArArO radical appear at 445 nm (is an element of = 2850 m(2) mol(-1)) and 730 nm (is an element of = 1680 m(2) mol(-1)). The radical pK(a) (HOArArO + H2O double left right arrow H3O+ + -OArArO) is measured to be 7.5 +/- 0.1. At pH 12, a reduction potential = 0.448 V us NHE for the -OArArO,H+/HOArArO- couple is obtained from measurements against the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) reference couple. Subsequently, from the parent pK(a) (9.4, 14.1) and the radical pK(a), the-reduction potential values in the pH range 0-14 are obtained. Some representative values (in V) are: 1.265 (at pH = 0), 0.805 (pH = 7.5), 0.616 (pH = 9.4), and 0.339 (pH > 14). From the reduction potential value at pH = 0, the upper limit of its:first O-H bond dissociation energy in the gas phase is estimated to be 357.7 kJ mol(-1).