The pathway of oxygen activation in oxidation reactions catalyzed either by transition metal or by gold catalysts has been extensively studied; however, this topic is still under discussion, and no consistent conclusion has been achieved up to date. One possible reason for this can be related to the fact that it is hard to find out whether the activated oxygen species come from the metal or the oxygen site. In an attempt to solve this problem, catalysts without oxygen (i.e. Fe-C3N4), or with controllable oxygen content (i.e. Au/AC), were tested in the oxidation of carbon monoxide in the gas phase and the oxidation of benzyl alcohol in the liquid phase. The results indicate that the metal site (i.e. Fe3+, Au) alone is not able to activate molecular oxygen and that the activity of the catalyst depends intimately on the amount of oxygen-containing species (e.g. surface groups of AC) that are used for oxygen activation. As a result, a scheme for the oxidation reaction is proposed, where the metal and oxygen sites of the catalyst are accounted for the adsorption and activation of substrate and molecular oxygen, respectively. (C) 2010 Elsevier Inc. All rights reserved.