The electrochemical oxidation of ionic liquids (ILs) in aqueous solutions on boron-doped diamond was investigated with the aim of explaining the influence of the IL composition (Cl-, Br-, BF4-, PF6-, CF3SO3-, CH3C6H4SO3-). By keeping the cation (1-butyl-3-methylimidazolium) constant and varying the anion, a comparative study of their oxidizability could be carried out. The experimental data suggest that hydroxyl radicals were the dominant oxidant reacting with the IL ionic pair. The chloride ion enhanced imidazolium cation degradation, whereas CH3C6H4SO3- competed with the cation for anodically produced hydroxyl radicals. The nature of the background anions (Na2SO4, NaCl, NaBr) also influenced the efficiency of the electrochemical process. The addition of trace halides further decreased chemical oxygen demand (COD). The results suggest that different oxidation mechanisms prevail at different halide concentrations. However, peroxodisulphate generated at the BDD anode in a Na2SO4 supporting electrolyte did not play a significant role in imidazolium salt oxidation. The intermediates found in the single compartment reactor after 3 h of electrolysis suggest that O-2(-center dot) also occured and governed in part the electrochemical oxidation of IL. (C) 2012 Elsevier B.V. All rights reserved.