Abstract The presence of Fe 3O 4–CoO/Al 2O 3 can improve degradation efficiency significantly during the ozonation of the herbicide 2-(2,4-dichlorophenoxy) propionic acid (2,4-DP). The main factors affecting degradation efficiency, such as pH, the catalyst concentration and addition of the scavenger, were investigated. The kinetics of the catalytic ozonation are also discussed. The results indicate that two factors, the oxidation after adsorption of 2,4-DP and the oxidation of hydroxyl radicals ( OH), lead to a great enhancement in ozonation efficiency during the catalytic ozonation of 2,4-DP in the presence of Fe 3O 4–CoO/Al 2O 3, in which the oxidation of the OH plays an important role. Under controlled conditions, the apparent reaction rate constants for the degradation of 2,4-DP were determined to be 2.567 × 10 −4 s −1 for O 3 and 1.840 × 10 −3 s −1 for O 3/Fe 3O 4–CoO/Al 2O 3. The results from the analysis of the reaction kinetics using the relative method showed that O 3/Fe 3O 4–CoO/Al 2O 3 possessed a larger R ct ( R ct is defined as the ratio of the OH exposure to the O 3 exposure, R ct = ∫ C OH t d t / ∫ C O 3 t d t ) than O 3, indicating that O 3/Fe 3O 4–CoO/Al 2O 3 produced more hydroxyl radicals.