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 O2 evolution reaction (OER) potential was observed with Nb/BDD compared to Si/BDD in both 0.05 M Na2SO4 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.