H(4)PVMo(11)O(40)(.)8H(2)O was studied during thermal activation as well as during isobulane oxidation by simultaneous in situ-EPR/UV-vis/Raman spectroscopy, in situ-FTIR spectroscopy, and quasi-in situ-H-1 and -V-51-MAS-NMR. In as-synthesized form, most V sites are pentavalent, octahedrally coordinated, and located within the intact Keggin anions. Stepwise dehydration in N-2 up to 350 degrees C leads to partial reduction and disintegration of the V sites from the Keggin units, followed by their condensation on the outer surface of the latter in square-pyramidal form. In water-free H4PVMo11O40, only V5+ (not V4+) is stable inside the Keggin unit. Thus, disintegration of V from the latter is favored by its reduction to the tetravalent state and thus depends on the redox properties of the atmosphere. Active sites in isobutane oxidation are most likely composed of single O4V4+/5+ = O species connected to Mo6+ via oxygen bridges. Partial deactivation occurs by formation of carbon-containing deposits. (c) 2006 Elsevier Inc. All rights reserved.