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Electrochemical oxidation using a boron-doped diamond (BDD) anode has been extensively investigated in various wastewater treatments, where the electrogenerated oxychlorides (ClO x –, x = 1 to 4) from Cl– oxidation have been well illustrated. However, the false chemical oxygen demand (COD) reduction induced by ClO x – adulteration has been almost completely overlooked when evaluating the electrochemical wastewater treatment performance. In this study, it was revealed that all the ClO x – species except ClO4 – served as strong oxidants in lieu of dichromate to degrade organic matter under digestion conditions during the COD test procedure. To eliminate the interference of ClO x – mixtures on the assessment of electrochemical COD removal performance, the modified sulfite-based reduction method was developed for the electrochemical case to remove ClO x – mixtures from the BDD-treated water samples before the COD test. It was demonstrated that the unadulterated COD removal performance of the BDD-based electrochemical system used for both synthetic and real wastewater treatments could be obtained after sulfite-based reduction of the electrogenerated ClO x – mixtures within 30 min in a sealed reaction chamber with the molar [sulfite]ini/[ClO x –]ini ratio of 1–3, where the reactants were isochronously depleted without producing oxidative radicals (SO4 •– and •OH). Due to extreme lack of oxygen in the sealed ClO x –-sulfite system, radical propagation reactions were terminated and ClO x – constituents (ClO3 –, ClO2 –, and ClO–) were reduced by sulfite stoichiometrically via the oxygen transfer process with Cl– and SO4 2– as the final products. This new sulfite-based protocol could be regarded as a robust and adulteration-proof countermeasure for evaluating the real COD removal performance of an electrochemical system.