Vanadium phosphate oxide (VPO) catalysts derived from VOHPO4 center dot 0.5H(2)O and VO(H2PO4)2 precursors showed improved rates in the catalysed liquid phase oxidation of p-cymene. Both catalysts showed high performance with conversions of up to 30% achieved within 3 h with a selectivity towards the tertiary cymene hydroperoxide (TCHP) of 75-80% when compared to lower conversions and poorer TCHP selectivity in the non-catalysed industrial oxidation processes that takes about 8-12 h reaction time. The temperature dependent structural changes during activation within the catalysts were studied by in situ XRD and TGA-MS. The in situ XRD showed that the VO(H2PO4)(2) form mainly an amorphous phase at temperature up to 600 degrees C while above 650-750 degrees C a stable VO(PO3)(2) phase was obtained. The in situ XRD studies of the (VO)(2)P2O7 phase derived from VOHPO4 center dot 0.5H(2)O showed to became more crystalline with increasing temperature from 400 to 750 degrees C. No formation of VOPO4 phases were observed in the activated (VO) 2P2O7 catalyst even at high temperature under the in situ XRD conditions.