Vanadium phosphorus oxide catalysts, used commercially for n-butane oxidation, are normally formed by heating VOHPO4 center dot 0.5H(2)O in a mixture of n-butane and air. The transformation of the precursor into the final catalyst is topotactic so that the crystalline habit is established when the precursor crystallises. In this study, the synthesis of VOHPO4 center dot 0.5H(2)O by reaction of a reduced suspension/solution of V2O5 in alcohol and o-H3PO4 has been studied by in situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) and focused ion beam microscopy (FIB), including cross-sectioning. XPS, XRD and microscopy evidence is presented for the temporal dissolution of V2O5 and formation in succession of VOPO4 center dot 2H(2)O; VOPO4 center dot H2O and VOHPO4 center dot 0.5H(2)O: Time-resolved in situ X-ray diffraction has identified VOPO4 center dot 2H(2)O at early synthesis times (less than 300 s) from d-spacings of d = 7.5 and 3.1 angstrom. The feature at 7.5 angstrom shifted to 6.7 angstrom during the first 20 min of synthesis, associated with the formation of VOPO4 center dot H2O and this phase collapsed as VOHPO4 center dot 0.5H(2)O formed. Thin symmetrical platelets of 10 x 10 mu m dimensions were observed when samples were recovered after short synthesis times. The platelets appeared to delaminate, possibly associated with the strain generated when the d-spacing shifted from 7.5 to 6.7 angstrom and the reducing action of the alcohol solvent. Growth of the VOHPO4 center dot 0.5H(2)O in the familiar rosette morphology occurs from these delaminated edges. Evidence is presented that VOHPO4 center dot 0.5H(2)O particles continue to grow and become more dense as synthesis time is increased. (C) 2004 Elsevier B.V. All rights reserved.