The aqueous ozonation of the herbicide mecoprop has been studied. A positive effect of pH and ozone partial pressure on the oxidation rate of mecoprop has been observed. Depending on the pH value, temperature has little or no influence on the oxidation rate probably due to the decrease of ozone solubility with the increase of this variable. The synergistic effect of ozone combined with hydrogen peroxide was also studied. Concentrations of hydrogen peroxide lower than 0.01 M do not affect the oxidation rate of mecoprop while higher concentrations inhibit the oxidation due to the consumption of both hydroxyl radicals and molecular ozone by hydrogen peroxide. From a mechanism of direct and radical reactions the rate constants of the reaction between ozone and mecoprop have been obtained. The mechanism has also been applied to the oxidation of mecoprop by ozone combined with hydrogen peroxide to determine the rate constant of the reaction between mecoprop and the hydroxyl radical. In all cases the importance of direct and radical pathways of mecoprop oxidation is discussed.