Flash photolysis of an aqueous solution of sodium metaborate and potassium peroxydisulphate at pH 11.5 gives rise to a new transient species with an absorption maxima at 590 nm (ε = 80 m2 mol-1) and 650 nm (ε = 90 m2 mol-1). The laser-Raman and 11B n.m.r spectra of metaborate solutions at pH 11.5 are consistent with the presence of the tetrahydroxyborate ion, B(OH)4-. The pKa and reduction potential of the radical B(OH)4 ., produced on flashing a mixture of B(OH)4- and S2O82- are found to be 10.75 ± 0.02 and + 1.4 V, respectively. The rate constants for the oxidation of several amines and phenols by this one-electron oxidant have been determined. Salt-effect studies are consistent with the reactive species being B(OH) 3O.-.