The catalytic role of VO(H2PO4)(2), the precursor of the VO(PO3)(2) phase, has been studied for n-butane oxidation to maleic anhydride. By comparison with the activated VPO catalyst, derived from the VOHPO4 . 0.5H(2)O precursor phase, VO(H2PO4)(2) gives a highly selective final catalyst. The total oxidation products CO and CO2 are not observed under any of the conditions examined, a result confirmed by extensive catalyst testing and carbon mass balances. The final catalyst derived from VO(H2PO4)(2) has a low surface area, ca. 1 m(2)/g, and consequently demonstrates low specific activity on the basis of n-butane conversion per unit mass. However, the intrinsic activity (activity per unit surface area) is found to be higher than that for catalysts derived from VOHPO4 . 0.5H(2)O. Since some VO(H2PO4)(2) is present in VOHPO4 . 0.5H(2)O, which is the precursor of the industrial catalyst, the results of this study complicate the simple model in which the (VO)(2)P2O7 phase derived from VOHPO4 . 0.5H(2)O is responsible for the selective oxidation of n-butane. The observation that the precursor VO(H2PO4)(2) can generate catalysts of high specific activity and of total selectivity to partial oxidation products might provide a useful insight into the design of a new series of high activity and high selectivity partial oxidation catalysts.