The electrogeneration of hydroxyl radicals was studied at a synthetic boron-doped diamond (BDD) thin film electrode. Spin trapping was used for detection of hydroxyl radicals with 5,5-dimethyl-1-pyrroline-N-oxide and with salicylic acid using electron spin resonance and liquid chromatography measurements, respectively. The production of hydrogen peroxide and competitive oxidation of formic and oxalic acids were also investigated using bulk electrolysis. The results have shown that oxidation of salicylic acid leads to the production of hydroxylated products (2,3- and 2,5-dihydroxybenzoic acids). These results demonstrate that the oxidation process on BDD electrodes involves hydroxyl radicals as electrogenerated intermediates. (C) 2003 The Electrochemical Society.