The effect of hydrogen peroxide as radical promoter in the wet air oxidation of phenol with activated carbon catalysts is studied in a trickle-bed reactor at 127 degrees C. 8 atm and 20-320 g(AC) h/g(Phenol). The modifications that hydrogen peroxide induces on the radical reaction mechanism and the consequences on the effluent ecotoxicity are analyzed. The synergistic effect between oxygen and hydrogen peroxide in the initiation step of the reaction is verified. Hydroperoxy radicals, produced by the reaction between hydrogen peroxide and adsorbed oxygen, initiate the reaction on the carbon and in the liquid phase. They are responsible for the increased initial activity which provokes a faster removal of phenol and aromatic intermediates and, therefore, of the toxicity. The adsorbed oxygen on the carbon is crucial for the efficient consumption of hydrogen peroxide. (C) 2009 Elsevier B.V. All rights reserved.