The behavior of hypochlorite ions and chlorinated organic compounds was investigated using model wastewaters to confirm the by-production of chlorinated organic compounds during the ozonation of wastewater which contains a high concentration of chloride ions such as leachate. An inhibition method for by-production of the chlorinated organic compounds was also studied. A hydrogen peroxide addition method was selected because it was expected to reduce the concentration of both dissolved ozone and hypochlorite ions. The following results were observed. (1) The concentration of the hypochlorite ions by-produced in the effluent increased with the increase of the chloride ion concentration in the influent and the ozone gas addition rate. The hypochlorite ion concentration increased in proportion to the value of t[O-3][Cl-]. (2) Inhibition of the hypochlorite ions by-produced during ozonation was achieved by dosing hydrogen peroxide. This effect was caused by the decreased generation rate that resulted from the decreased concentration of dissolved ozone. The disappearance reaction of hypochlorite ions had almost no effect. (3) The concentration of chlorinated organic compounds by-produced in the effluent increased with the increased concentration of chloride ions. Considering the increased concentration of hypochlorite ions and the decreased concentration of dissolved ozone which is resulted from the increased concentration of chloride ions in the influent, by-production of chlorinated organic compounds was caused by the reaction of the hypochlorite ions and organic carbon in the influent. (4) A reduction rate of 61-69% for the chlorinated organic compound concentration in the effluent was obtained by dosing hydrogen peroxide. Hydrogen peroxide inhibited by-production of chlorinated organic compounds. This was caused by the decreased concentration of dissolved ozone which leads to by-production of hypochlorite ions and chlorinated organic compounds. Such a decrease was also observed in UV/O-3 combined treatment. These results indicate that, considering the behavior of chlorinated organic compounds, ozonation of wastewaters containing a high concentration of chloride ions such as leachate should be done with hydrogen peroxide dosage.