A leachate from a municipal landfill and an industrial wastewater from an emulsion-breaking unit have been treated by chemical wet oxidation using ozone and hydrogen peroxide as oxidants. The best results were obtained with UV radiation resulting in the formation of OH. radicals. If ozone is used, mass transfer will be the limiting step in the degradation reactions. The specific consumption of hydrogen peroxide is drastically reduced if pure oxygen or air is dispersed into the wastewater. Under optimum conditions, the COD of the leachate and of the industrial wastewater could be reduced by 90% in c. 3 and 5 h, respectively.