Abstract: The degradation of 2,4-dichlorophoxyacetic acid (2,4-D)by using ozonation alone（O₃） or ozonation combined with sonication(O₃/US)was studied.The results showed significant synergistic effect of ozone and ultrasound.•OH radical played an important role in the degradation of 2,4-D by the O₃/US technique.Under ultrasound irradiation,mechanical effects of ultrasonic energy enhanced the mass transfer of gaseous ozone to the liquid phase.The enhancement of ultrasound was mainly attributed to that ozone decomposition could be accelerated to produce abundant radicals by cavitation effect, which resulted in high temperatures and pressures during cavitation bubbles collapse.