V/P/O-based catalysts were prepared by thermal treatments of VOHPO(4)0.5H(2)O precursors prepared with the organic procedure. Different methods for precursor dehydration led to compounds which were characterized by the prevailing presence of crystalline (VO)(2)P2O7, but which also contained either V5+ species or V3+ species. The catalytic performance of these compounds in n-butane oxidation under almost-equilibrated conditions was compared. It was found that the presence of either V3+ or V5+ enhances the specific activity in n-butane oxidation, while the selectivity to maleic anhydride at low n-butane conversion (30%) remains substantially unaffected. A fully equilibrated, well-crystallized (VO)(2)P2O7 was reduced with H-2. The reduced compound was more active than the fully equilibrated vanadyl pyrophosphate, while exhibiting comparable selectivity to maleic anhydride.