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The electrochemical performance of thin film boron-doped diamond (BDD) electrodes deposited on either single crystal p-type Si (Si/BDD) or Nb (Nb/BDD) substrates have been investigated for the electrooxidation of azo-dye Acid Violet 7 (AV7). Both electrodes were characterized by linear voltammetry and the production of hydroxyl radicals ( • OH) at their surfaces was assessed by spin trap experiment using N,N-dimethyl-p-nitrosoaniline (RNO) as a spin trap molecule. Slightly higher O 2 evolution reaction (OER) potential was observed with Nb/BDD compared to Si/BDD in both 0.05 M Na 2 SO 4 and AV7 solutions. Enhanced bleaching of RNO was always observed when an increased current density was applied and a significant decay of the adsorption band of RNO was attained with Nb/BDD compared to Si/BDD electrode, indicating higher generation of • OH on its surface. Complete decolorization of 200 mg L -1 of AV7 was achieved with either electrodes at current density of 30 and 60 mA cm -2 in 120 min or less. Quite similar degradation efficiency was attained at both electrodes but Si/BDD showed better results than Nb/BDD. Nevertheless, lower energy requirements was achieved by Nb/BDD, demonstrating that it can be a suitable electrode for an efficient electrochemical oxidation process.