Many effective technologies have been developed and approved for ballast water treatment, and the effectiveness of a process should be evaluated considering its environmental friendliness. The aim of this study was to evaluate the formation and formation mechanisms of bromate (BrO₃-) and chlorate (ClO₃-) in seawater during ozonation, electrolysis, and a combined (ozonation/electrolysis) process. In seawater ozonation, BrO₃- was generated exceeding a 5 mg/L ozone dose despite the high Br- (65 mg/L). The formation of BrO₃- and ClO₃- by electrolysis depended on the electrode materials where two types of electrodes (IrO₂/Ti and Pt/Ti) were used. The combined (ozonation/electrolysis) process generated much higher levels of BrO₃- and ClO₃- than either individual process. In ozonation or electrolysis, mixed oxidant including HOCl/OCl- and HOBr/OBr- is the main parameter for inactivation, which is called total residual oxidant, TRO. In this study, a predictive model for BrO₃- and ClO₃- was suggested in terms of TRO formation. This predictive model can recommend allowable TRO (generated by ozonation or electrolysis) for practical applications in seawater treatment.