Bulk crystallinity of vanadyl(IV) pyrophosphate catalysts for n-butane partial oxidation increased up to 23 days on stream as determined by XRD and Raman spectroscopy, while selectivity reached steady state after 8-10 days. Electron microscopy detected a 15 Angstrom amorphous layer terminating the (200) planes of (VO)(2)P2O7 in fresh catalysts that was not observed in the equilibrated catalysts. It is suggested that ordering of (200) planes at the surface of (VO)(2)P2O7 is responsible for selective oxidation.