The kind and percentage of point defects in commercial as well as laboratory prepared hemihydrate VPO precursors have been analysed using a combination of electron diffraction experiments and diffraction pattern simulation. Experimental patterns reveal the systematic existence of kinematically forbidden reflections that are attributed to vacant lattice sites in the VOHPO4 center dot 1/2H(2)O matrix. The vacancies are associated with vanadium ions and are distributed into the bulk and surface of hemihydrate in an ordered fashion. The amount of vacancies is in similar proportion between the two precursors studied. A considerable enhancement in the forbidden reflections intensity in doped hemihydrate samples has been experimentally observed, which is attributed to the partial substitution of dopant ions into V vacant sites. The beneficial effect of such a modified hemihydrate structure on the resulting VPO catalyst performance is demonstrated, since the facilitation of promoter incorporation in V-based vacant active sites brings upon an enhanced activity and selectivity for the catalyst. (C) 2009 Elsevier B.V. All rights reserved.