Sulfamethoxazole (SMX) was used as a model Substrate for electrochemical oxidation at a boron-doped diamond anode in the presence of chloride ion, which is present in many waste streams. In the absence of chloride, oxidation of SMX involves mineralization, an electrochemical advanced oxidation process (EAOP) that is initiated by attack of anode-derived hydroxyl radicals. The rate of disappearance of SMX increased monotonically upon addition of chloride ion but without inhibiting mineralization in the early stages of oxidation. This demonstrated that electrochemical hypochlorination (EH) and EAOP are not mutually exclusive reaction pathways; products of EH can undergo EAOP and vice versa. Persistent chlorinated byproducts were formed in the presence of chloride ion, indicating that chloride is a significant detriment to the success of EAOP. No mineralization was observed upon chemical hypochlorination of SMX with sodium hypochlorite.