The oxidation of hazardous organic compounds leads to the formation of several by-products, being oxalic acid and oxamic acid final oxidation products refractory to ozonation. The present work aimed to study the ozonation of those carboxylic acids in the presence of activated carbon at different solution pH. For comparative purposes, experiments of adsorption on activated carbon, ozonation, and ozonation in the presence of activated carbon were carried out. In order to clarify the reaction mechanism, some experiments were done in the presence of a radical scavenger. With the aim of evaluating the role of the activated carbon surface chemistry during the ozonation, two activated carbon samples were assessed. A significant synergistic effect between ozone and activated carbon was evidenced in the oxidation of oxalic acid. Oxamic acid was found to be refractory to oxidation at pH 7. On the other hand, at pH 3, the mineralization of oxamic acid was significantly enhanced by the presence of activated carbon. Generally, the presence of activated carbon during ozonation increased the rate of degradation of both carboxylic acids leading to mineralization. Best results were achieved with the most basic activated carbon. In both cases, the efficiency of activated carbon promoted ozonation decreases with the increase of solution pH. (C) 2007 Elsevier B.V. All rights reserved.