DuPont's vanadium phosphorous oxide catalyst (VPO) deactivated with time-on-stream in a commercial butane to maleic anhydride reactor. Coincidentally, V5+ phases formed on the surface (based on XPS) - principally beta-VOPO4 but also V2O5. This catalyst was reactivated by introducing a small amount of a VPO (theoretical P/V atomic ratio = 0.86) phase. The maleic anhydride production rate of the reactivated catalyst was higher by about 60% compared to the used catalyst. n-Butane conversion increased by about 50% and the selectivity to maleic anhydride improved by 15%. The analyses of the modified catalyst by X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy showed that the V2O5 and beta-VOPO4 phases disappeared and suggested that an amorphous phase formed on the surface. The treatment resulted in a lower V5+/V4+ and P/V ratios on the used catalyst surface. (C) 2012 Elsevier B. V. All rights reserved.