The synthesis of VOHPO4 . 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 (FIB) microscopy, including cross-sectioning. XPS, XRD, and microscopy evidence is presented for the temporal dissolution of V2O5 and formation of VOPO4 . 2H(2)O, VOPO4 . H2O, and VOHPO4 . 0.5H(2)O. The XPS technique allows us to trace the development of surface vanadium, oxygen, and carbon states as well as the surface P:V ratio. Four vanadium species were identified. The oxygen vacancies on the surface were characterised by the V 2p(3/2) peak at 515.5 eV and by the O 1s peak at 531.2 eV. V2O5 exhibited the V 2P(3/2) peak at 517.4 eV and the O 1s peak at 530.0 eV. The dihydrate phase VOPO4 . 2H(2)O and hydrate phase VOPO4 . H2O were monitored by the V 2P(3/2) peak at 518.1 eV and the O 1 s peak at 531.2 eV. The VPO catalyst precursor VOHPO4 . 0.5H(2)O, the V4+ oxidation state, shows the V 2p(3/2) peak at 516.6 eV and the O 1 s peak at 531.2 eV. The O 1s peak at 532.9 eV is assigned to crystal water. In situ monitoring of the synthesis by XRD was in a good agreement with the ex situ XPS analysis. VOPO4 . 2H(2)O and VOPO4 . H2O were successfully identified by XPS as a metastable phase, which forms at short synthesis times. As the hydrate phase concentration decreases the concentration of VOHPO4 . 0.5H(2)O increases. All XPS data were consistent with the earlier proposed mechanism, which supposed that VOPO4 . 2H(2)O dehydrates to VOPO4 . H2O, delaminates and the delaminated edges of VOPO4 . H2O serve as the nucleation point for growth of VOHPO4 . 0.5H(2)O. (C) 2004 Elsevier Inc. All rights reserved.