Abstract: In order to investigate the kinetics of direct oxidation of 2,4-dichlorophenoxyacetic acid by ozone molecule, the oxidation of 2，4-D by ozonation was studied in a laboratory scale reactor.2,4-D dissolved in water was dissociated to deprotonized and protonated species. The results showed the dependence of second-order rate constants on the degree of dissociation. A kinetics model in view of dissociation was proposed to predict overall 2,4-D degradation for different pH values and was proved to be good for practical degradation.