The structural genesis of a M00.68V0.23W0.09 oxide with Mo5O14-like structure has been examined. A precursor prepared by spray-drying of mixed aqueous metal salt solutions was calcined in air and subsequently treated in helium at different temperatures. X-ray diffraction, HRTEM, V-51 magic angle spinning (MAS) NMR, ESR, UV/vis DR spectroscopy and oxygen and hydrogen adsorption measurements have been applied to monitor the preparation procedure. It was found that a structure closely related to that of Mo5O14 already appears at nano-scale level after calcination of the spray-dried precursor in air at 350 degrees C. At this stage, the material comprises of crystalline particles less than 3 nm in size stabilized by an amorphous matrix. Further heating causes nanostructural rearrangements that lead to the formation of the final Mo0.68V0.23W0.09 oxide with phase-pure polycrystalline structure. Molybdenum and tungsten ions are hexavalent and coordinated in an octahedral environment. Furthermore, vanadium is present as V4+ and V5+, ions occupying octahedral and highly distorted trigonal pyramidal sites, respectively. According to the results of H-2 and O-2 adsorption the crystalline ternary oxide does not possess accessible micropores. Oxygen pulses at 450 degrees C and reductive treatment with pure hydrogen at 300 degrees C did not cause noticeable changes of the bulk structure thus indicating a remarkable structural stability of the complex MoVW oxide under redox conditions at elevated temperature. (c) 2007 Elsevier B.V. All rights reserved.