The oxidation states of vanadium on the surface of VPO catalysts which show the characteristic diffraction pattern of vanadyl pyrophosphate have been investigated by X-ray photoelectron spectroscopy (XPS). The (VO)(2)P2O7 phase is obtained from crystalline precursors which show the X-ray pattern of VOHPO4 . 1/2H(2)O. The XPS spectra of 40 solids (precursors, catalysts, and model compounds) hinted the presence of low concentrations of V-V centers on the surface of the bulk V-IV catalysts, To further investigate this matter, the solids were oxidized or reduced in a pretreatment chamber attached to the spectrometer. These studies confirmed the presence of V-V centers on the surface of the catalysts which may be initiated by strong oxygen chemisorption on exposed V-IV sites of (VO)(2)P2O7. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to beta-VOPO4, a phase which has a deleterious effect on catalyst performance.