X-ray diffraction, Raman spectroscopy, P-31 MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending on both the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence of a number of micro-crystalline and amorphous V(IV) and V(V) phases not evident by XRD. The presence of VOPO4 phases was detrimental to the performance of the VPO catalysts for n-butane oxidation. The best model organic VPO catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO, phase impurity. Raman spectroscopy detected vanadyl metaphosphate, VO(PO3)(2), in the catalysts derived from aqueous precursors possessing P/V ratios greater than 1. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. P-31 NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VPO catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.