Redox properties of the surface of vanadyl pyrophosphate ((VO)(2)P2O7) have been investigated mainly by means of in situ Raman spectroscopy as well as IR measurement of low temperature adsorption of CO. The results of in situ Raman spectroscopy were consistent with our previous conclusion [G. Koyano, T. Okuhara, M. Misono, J. Am. Chem, Sec., 120 (1998) 767.] that the reversible oxidation and reduction between (VO)(2)P2O7 and X-1 phase of VOPO4 operates on the surface during the selective oxidation of n-butane with a relatively high selectivity for maleic anhydride, and showed that the surface X-1 phase formed by oxidation was reversibly reduced to the original (VO)(2)P2O7 phase. However, beta-VOPO4 was once formed on the surface, the surface and the selectivity was not recovered by the reduction. It was confirmed by the IR investigation that the change of the surface were essentially reversible for the oxidation and reduction process between (VO)(2)P2O7 and X-1 phase, but the oxidation to beta-VOPO4 changed irreversibly the surface. Furthermore, it was indicated that n-butane preferentially interacts with a Lewis acid site than a Bronsted acid site.