Spectroscopic techniques (X-ray absorption, Raman, and UV−visible) were utilized to monitor the effect of adsorbed water, calcination temperature, and reduction in H₂ on the structure of dispersed VO_x for vanadia supported on SiO₂, Al₂O₃, TiO₂, ZrO₂, and HfO₂ prepared with VO_x surface densities ranging from 0.46 VO_x/nm² to 11.1 VO_x/nm². Supported vanadia was found to exist as monovanadate, polyvanadate, or V₂O₅ species, the distribution among these species depending on the support for a given VO_x surface density. Dehydration resulted in the appearance of monovanadate species on all supports, with the extent of these species decreasing in the order HfO₂ > Al₂O₃ > ZrO₂ > TiO₂ > SiO₂. Hydration of the samples caused a decrease in the monovanadate species and a slight increase in polyvanadate species. Oxidation at elevated temperature resulted in an increase in the crystallinity of V₂O₅ present on SiO₂, a conversion of V₂O₅ into polyvanadate species on Al₂O₃, and the appearance of mixed-metal oxide phases on TiO₂, ZrO₂ (ZrV₂O₇), and HfO₂ (HfV₂O₇). The appearance of an interaction between vanadia and titania coincides with the transformation from anatase to rutile TiO₂. ZrV₂O₇ and HfV₂O₇ are postulated to form via the interaction of surface VO_x species with the support as the support begins to undergo a phase transition from tetragonal to monoclinic. H₂ reduction produced limited changes in the structure of dispersed vanadia except on Al₂O₃, where V₂O₅ was transformed into polyvanadate species.