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Three vanadyl pyrophosphate catalysts synthesised via VOPO4·2H2O have been calcined for 40, 100 and 132 hours at 673 K in a reaction flow of n-butane/air. These catalysts were denoted VPD40, VPD100 and VPD132. The nature of oxidants in/on these catalysts has been investigated by using temperatureprogrammed techniques. One peak maximum was found desorbed in O2 temperature programmed desorption (TPD) at 992 K for VPD40 with a total amount of oxygen desorbed of 2.4×1020 mol×g-1. However an increased of calcination duration to 100 and 132 hours produced one oxygen peak desorbed at 986 K with a total amount of oxygen desorbed doubled to 4.4×1020 mol×g-1. Temperature programmed reduction with H2 resulted in the removal of ~ 10 times more than the lattice oxygen which can be desorbed thermally with the development of a new low reduction peak at ~ 840 K.