Time-resolved in situ X-ray diffraction was used to study the formation of the vanadium phosphorous oxide (VPO) catalytic precursor phase VOHPO4 . 0.5H(2)O in organic media. In situ energy-dispersive X-ray diffraction has identified a new phase at early reaction times (less than 300 s) at d-spacings of 7.5 and 3.1 Angstrom. At short reaction times, thin symmetrical platelets of 10 by 10 mum dimensions were observed. Scanning Electron Microscopy combined with energy-dispersive X-ray analysis (EDXA) indicated that these platelets contained vanadium and phosphorus. The features at 7.5 Angstrom, which shift to 6.7 Angstrom during the first 20 min of synthesis, are associated with VOPO4 . 2H(2)O and VOPO4 . H2O and disappear as VOHPO4 . 0.5H(2)O forms. The platelets appeared to delaminate, possibly associated with the strain generated when a d-spacing shifts from 7.5 Angstrom to 6.7 Angstrom, from which epitaxial growth of the VOHPO4 . 0.5H(2)O into the familiar rosette morphology occurs.