The molecular structures and oxidation states of supported 1-5% V2O5/ZrO2 catalysts during propane oxidative dehydrogenation (ODH), with varying propane/O-2 ratios, were examined by in situ UV-vis-NIR diffuse reflectance and in situ Raman spectroscopic studies. The results indicate that the reduction extent of surface V5+ cations to V3+/V4+ cations under steady-state reaction conditions increases with the propane/O-2 ratio. At the same propane/O-2 ratio, the relative extent of reduction of the supported V2O5/ZrO2 catalysts generally increases with the surface vanadia loading, and the polymerized surface VO4 species are more extensively reduced than the isolated surface VO4 species during steady-state propane oxidation. The reactivity studies reveal that at the same reaction conditions, both polymerized and isolated surface V cations are active sites for propane oxidation and that the specific catalytic reactivity (as measured by turnover frequency; TOF) is independent of the surface density of the two-dimensional vanadia overlayer on the ZrO2 support. Furthermore, the relatively constant TOF with surface vanadia coverage demonstrates that propane ODH to propylene requires only one surface VO4 site. However, the propylene selectivity increases with increasing surface vanadia loading due to the removal of nonselective surface sites, possibly terminal Zr-OH groups, on the ZrO2 surface by the deposition of surface vanadia species. The propane/O-2 ratio greatly affects the selectivity of these catalysts. Highly oxygen-rich environments (e.g., propane/O-2 ratio= 1/10) give rise to the highest propylene selectivity, revealing that propylene production is favored on highly oxidized surface vanadia (+5) sites. Small V2O5 crystallites above monolayer surface vanadia coverage do not contribute to propane ODH because of their low dispersion and low number of active surface sites (spectator vanadia species).