Advanced oxidation of atrazine (ATZ) via electrolysis and ozonation separately and in combination was studied in the presence of concentrated chloride and bromide electrolytes at pH 7. Ozone (O3) was bubbled at a rate of 0.3 L/min into a batch electrolysis reactor utilizing a boron-doped diamond (BDD) anode and stainless steel (SS) cathode operating at a current density of 10 mA/cm2. Results from separate experiments in a 54 mM sodium chloride (NaCl) or 54 mM sodium bromide (NaBr) electrolyte were compared with data previously reported by the authors produced in the same system utilizing a sulfate (SO4 2−) electrolyte. Bromide and chloride slightly increased the rate of ATZ degradation compared to sulfate when electrolysis only was applied. When ozone was applied individually, the estimated first-order degradation rate of atrazine decreased 20% with chloride and 87% with bromide compared to sulfate. The degradation rate of ATZ decreased 86% with chloride in the combined electrolysis and ozonation mode and 93% with bromide, both as compared to sulfate. This is significant since combined electrolysis and ozonation has been shown in previously published works to produce strong synergism for contaminant removal in sulfate. The proposed mechanism for antagonism with a reactive electrolyte such as chloride or bromide is increased competition in the system for hydroxyl radicals (HO%). Free halogen species and bromide are reactive with HO%, decreasing the likelihood of direct reaction between HO% and ATZ. In addition to the antagonism, all cases except ozonation in chloride generated either bromate (BrO3-) or chlorate (ClO3−).