In-situ UV-visible spectroscopy was used to measure the extent of reduction of active centers in VOx/gamma-Al2O3 during oxidative dehydrogenation (ODH) of propane. Prevalent extents of reduction (0.062 to 0.30 e(-)/V) are much smaller than required for the formation of stoichiometric V3+ or V4+ suboxides. Surface oxygen atoms are the most abundant reactive intermediates during propane ODH, as previously suggested by kinetic and isotopic studies. These measurements involved the rigorous calibration of UV-visible intensities in the pre-edge region using quantitative reoxidation of a small number of centers reduced in H-2. Transients observed during changes in C3H8 and O-2 concentrations indicate that only a fraction of the prevalent reduced centers (similar to30-40%) are active in catalytic turnovers, while the rest are reoxidized in time scales much longer than turnover times. The number of catalytically relevant reduced centers depends only on C3H8/O-2 ratios, and not on individual reactant concentrations, indicating that oxygen vacancies are the predominant reduced centers and that hydroxyls and alkoxides are present at much lower concentrations. The fraction of V-atoms that exist as catalytically reduced centers and the rate of propane ODH (per exposed V-atom) increase with increasing vanadia surface density and domain size up to surface densities typical of polyvanadate monolayers (similar to7.5 V/nm(2)) and then reach nearly constant values at higher surface densities. This relation between the extent of reduction during catalysis and the propene formation rates confirms the redox nature of catalytic cycles and the exclusive kinetic relevance of the reduction part of the cycle, in which C-H bonds are activated using lattice oxygen atoms. This method for measuring the extent of reduction during catalysis using pre-edge features in the UV-visible spectrum provides greater sensitivity and time resolution than X-ray absorption and UV-visible spectroscopic methods based on near-edge spectral features. The approach and initial results seem generally applicable to oxidation reactions using lattice oxygens as reactive intermediates.