Full text can be accessed at https://www.jstage.jst.go.jp/article/jcej/40/4/40_4_378/_articleThe degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was conducted by ozonation, TiO2/UV treatment only, and two-stage treatment of ozonation followed by TiO2/UV treatment. In the case of ozonation, 2,4-D could be degraded and vanished within about 10 min at pH 9 and 20°C, but the TOC value was reduced by only about 40% by 30 min ozonation. In the case of TiO2/UV treatment, about 5 d was needed for complete disappearance of 2,4-D at pH 5 and 20°C. On the other hand, when the two-stage treatment of ozonation plus TiO2/UV treatment was applied, 90% decline of TOC was achieved within about 20 h, i.e., 30 min ozonation and 20 h TiO2/UV treatment thereafter. Furthermore, 90% of chloride ions in 2,4-D were liberated by the two-stage treatment. From a bioassay test of the two-stage treated water by the use of an osteoclasts of goldfish scales or Daphnia magna, it was found that 2,4-D was degraded into harmless substances with little estrogen activity and toxicity.