The activation of oxygen is a key step for the selective oxidation of benzene to phenol and the reason is discussed. The active oxygen species is produced from molecular oxygen in a so-called "reductive activation" process. The vanadium oxide supported on alumina was pre-reduced by hydrogen or ascorbic acid to lower valence vanadium species acting as reduction source and the activity is investigated in the reaction. It is found that the V4+ valence vanadium (VO2+) is effective for the reaction from the characterization of catalysts by XPS.