A Pd-V2O5/Al2O3 catalyst system was studied by means of X-ray diffraction methods, chemical analysis and electron spin resonance spectroscopy. Under working conditions of the reaction of complete oxidation of benzene, the following phases were found : beta-NaxV2O5, theta-Al0.33V2O5 and Al0.07(3+) V0.07(5+) V1.86(4+) O4. An efficient delocalization of electrons between clustered V4+ ions in these phases facilitates the electronic transitions in the redox steps of the catalytic oxidation.