The nature of VOx sites in mixed oxides of supported VOx (on both pure oxide and mixed oxide supports), molecular sieves, zeolites, clays, hydrotalcites, stochiometric bulk oxides and bulk solid solutions were investigated. For supported metal oxides, zeolites and molecular sieves, the VOx species are exclusively present as surface VOx phases below monolayer coverage or the maximum dispersion limits. For layered clays and hydrotalcites, the VOx is present in the hydroxide layers at modest temperatures and react with the clays and hydrotalcites at higher temperatures (>350 degrees C) when their layered structures decompose. Surface VOx species are always also present for bulk oxides and bulk solid solutions. The rapid diffusion kinetics of VOx, due to its low Tammann temperature, coupled with the lower surface free-energy of vanadium oxide are responsible for the universal presence of surface VOx sites on all mixed oxide materials. Furthermore, surface reactivity studies demonstrate that the surface VOx sites are the catalytic active sites for all V-containing mixed oxide catalytic materials. (C) 2010 Elsevier B.V. All rights reserved.