Abstract: The degradation of 2,4-dichlorophoxyacetic acid (2,4-D) by ozonation combined with sonication(O₃/US) was investigated.Based on the authors’ first paper about the synergistic effect of ozone and US, this paper presents intermediate products of 2,4-D degradation.The main intermediates were 2,4- dichlorophenol, benzopuinone, hydroquinone and organic acids,such as maleic acid and oxalic acid and so on.In various experimental conditions, concentrations of main intermediates and chloride ion in the solution were determined to analyze reaction pathways, and the degradation mechanism was discussed.