Vanadium-phosphorus-oxides catalysts for butane oxidation to maleic anhydride were prepared in an organic medium in pilot-scale equipment and were tested in a single-tube packed bed reactor. To avoid reactor thermal runaway, a nonuniform catalyst loading was adopted to moderate the reactor temperature. The precursors and catalysts were characterized by average oxidation states of vanadium, PN atomic ratios, X-ray diffraction and pore size distribution. VOHPO4. 0.5H(2)O and (VO)(2)P(2)O7 . 9H(2)O in precursors, (VO)(2)P2O7 and alpha-VOPO4 in catalysts were observed in the present work. After completion of a long run catalytic test, the used catalysts were carefully sectioned and removed from the reactor for characterization. The axial profiles of average oxidation states of vanadium and PN atomic ratios were obtained. The average oxidation states of vanadium were between 4.0 and 4.3 for the used catalysts and were related to the axial profiles of temperature obtained during the test. The PN ratios of the used catalysts were in the range of 1.07-1.08, almost the same as that of the precursor. No phosphorus loss was observed after a long-run test and this enabled the catalyst to maintain a long useful life. The long-run catalytic test results showed that the catalyst was of high activity and good selectivity for butane oxidation to maleic anhydride.