The oxidation behavior of chloride ions on platinum electrodes was investigated in a natural ionic liquid, 1-butyl-3-methyl-limidazolium tetrafluoroborate (BMIMBF4) in the presence of high concentrations of 1buty1-3-methyl-limidazolium chloride (BMIMCI). Analysis of the electrode reaction was explored using cyclic voltammetry, and chronoamperometry with a platinum micro-disk electrode, and bulk potentiostatic electrolysis and UV-vis spectroscopy. The anodic oxidation of chloride ions on the platinum micro-disk electrode in the mixture was considered to be an irreversible process with diffusion controlled as revealed by cyclic voltammetry. The diffusion coefficient, D, and the number of electrons transferred, n, for anodic oxidation of Cl- in BMIMBF4 derived from results of chronoamperometry revealed that the oxidation of chloride ions was more likely to form tri-chloride ion, Cl-3(-) but not chlorine, Cl-2. Bulk electrolysis and UV-vis spectroscopy further confirmed that the tri-chloride ion was the main product from the overall oxidation of the chloride ion. (C) 2013 Elsevier Ltd. All rights reserved.