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This study reports the availability of seawater as an economical chloride (Cl − ) source for the UV/ electro-chlorine process from the viewpoints of advanced oxidation performance and unwanted byproduct formation of chlorate ion (ClO 3 − ) and bromate ion (BrO 3 − ). In the electrochemical oxidant production stage, the oxidant production rate in diluted seawater containing 30 mM Cl − was 21% lower than that in the NaCl solution due to coexisting electrolytes in the seawater. The ClO 3 − formation during electrolysis was successfully inhibited under acidic conditions and BrO 3 − formation was not detected in the diluted seawater. However, ClO 3 − and BrO 3 − were steadily formed in the undiluted seawater electrolysis, even when the initial pH value was set to 3. The oxidant utilization efficiency for 1,4-dioxane removal during UV irradiation was not deteriorated in the diluted seawater but decreased under basic conditions due to the radical scavenging effect of electrochemically produced free bromine and free chlorine. As a result, the formation of BrO 3 − and ClO 3 − was enhanced under basic conditions, whereas BrO 3 − formation was completely inhibited at an initial pH ≤ 5. Consequently, the diluted seawater was thought to be available as a Cl − source for the UV/electro-chlorine process if an acidic condition was maintained throughout the operation.