The reduction-oxidation of copper ions in Cu-Pd-H3PO4/SiO2 catalysts, active in the direct oxidation of benzene to phenol, was studied by means of electron spin resonance spectroscopy. The isolated and aggregated copper (II) ions were found to exist on the Cu-Pd-H3PO4/SiO2 catalyst after evacuation at room temperature. A portion of the Cu(II) ions supported on SiO2, isolated Cu(II) ions coordinated with PO43-, was found to be reversible with alternate reduction-oxidation treatments at 200 degrees C. When the ESR results were compared with the formation rates of phenol, the ＇reversible＇ Cu(II) ions were concluded to be responsible for catalytically active sites.