The effect of preozonation on activated sludge effluent before activated carbon treatment was evaluated in terms of its effect on the adsorption process. It was found that the total removal of organic substances by the combined action of ozone and activated carbon was superior than by either ozone or activated carbon separately. Although ozonation leads to a decrease in the adsorbability of organic substances, the overall removal of organic substances is not impaired except during the early stages of operation before an effective biogrowth is established. Preozonation extends the operational life of activated carbon. This is explained, in part, by improved desorption and biodegradation of the adsorbate.