The surface structure of vanadyl pyrophosphate ((VO)(2)P2O7) and its changes upon controlled oxidation and reduction have been investigated comprehensively by means of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), EXAFS, X-ray diffraction (XRD), transmission electron diffraction (TED), and "micropulse" reaction of n-butane. Oxidation of a well-defined (VO)(2)P2O7 With O-2 (1 atm) at 733 K formed "X-1 phase" as a thin surface overlayer on (VO)(2)P2O7, where X-1 phase is a phase reported previously (Shimoda, T.: Okuhara, T.; Misono, M. Bull. Chem. Sec. Jpn. 1985, 58, 2163-2171) and similar to delta-VOPO4. By the repeated micropulse reactions of n-butane, the surface X-1 phase was gradually reduced back to (VO)(2)P2O7, showing that reversible redox reactions between X-1 phase and (VO)(2)P2O7 occur by the reactions with butane and oxygen. XRD,:EXAFS, and TED demonstrated that X-1 phase has a structure analogous to (VO)(2)P2O7, in both of which V-O-V I,air sites exist. The micropulse reaction of n-butane with the surface X-1 phase produced maleic anhydride with a significantly higher selectivity (similar to 40%) than that with beta-VOPO4 (<10%). These results indicate that X-1 phase is the real active phase involved in the redox cycle for the oxidation of n-butane to maleic anhydride over (VO)(2)P2O7.