Oxidation and reduction processes of the surface of vanadyl pyrophosphate ((VO)(2)P2O7) have been studied by means of Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry, and "micro-pulse" reaction of n-butane in relation to the selective oxidation mf n-butane. Oxidation of (VO)(2)P2O7 by O-2 was carried out to controlled extents by changing the reaction temperature and period. When (VO)(2)P2O7 was oxidized at 733 K for 2 h, about one layer of the surface of (VO)(2)P2O7 was oxidized to the X(1) phase, which was reported previously by the authors, as deduced from the amount of O-2-uptake and spectroscopic changes. Reductions of the surface X(1) phase and beta-VOPO4 were carried out by a "micro-pulse" reaction with n-butane at 733 K. The reduction by n-butane changed the surface X(1) phase to (VO)(2)P2O7 and, at the same time, gave maleic anhydride with selectivities ranging from 13 to 40%. The selectivity was less than 10% in the case of beta-VOPO4. These results suggest that the redox cycle between the X(1) phase and (VO)(2)P2O7 in the surface layer is involved in the catalytic oxidation of n-butane to maleic anhydride.