The effects of MoOx structure on propane oxidative dehydrogenation (ODH) rates and selectivity were examined on Al2O3- supported molybdenum oxide catalysts with a wide range of Mo surface density (0.4-12 Mo/nm(2)). X-ray diffraction and Raman, UV-visible, and X-ray absorption spectroscopies showed that the structure of dispersed molybdena depends strongly on the Mo surface density. Two-dimensional MoOx oligomers formed preferentially for Mo surface densities below 4 Mo/nm(2). At higher surface densities, these MoOx oligomers coexist on Al2O3 surfaces with three-dimensional MoO3. UV-visible edge energies decrease with increasing Mo surface density, consistent with the growth of MoOx structures. The evolution of near-edge spectral features in the X-ray absorption spectra and the gradual appearance of a Mo-Mo scattering peak in the radial structure function confirmed the growth of MoOx domains with increasing surface density. ODH rates per Mo atom increased with increasing Mo surface density and reached a maximum value for samples with similar to4.5 Mo/nm(2); this behavior reflects an increase in the reactivity of surface Mo species, because all MoOx species are exposed at domain surfaces in this surface density range. As also shown for VOx-based catalysts, turnover rates are higher on two-dimensional domains than on isolated monomers and they increase as the MoOx domain size increases. The rates of reduction of MoOx species in H-2 or C3H8 were probed using kinetic and X-ray absorption methods; these reduction rates increased in parallel with ODH rates as the MoOx surface density increased, apparently as a result of the ability of larger domains to delocalize the higher electron density that accompanies the reduction process. As the surface density increased above 4.5 Mo/nm(2), ODH rates (per Mo atom) decrease, as a result of the loss of accessibility caused by the formation of MoO3 crystallites. For these latter samples, the ODH rate per BET surface area approached a constant value as the surface density increased, because all exposed surfaces in these samples reside within two- or three-dimensional MoO3 structures with similar reactivity. The ratio of rate constants for propane ODH and propane combustion reactions increased with increasing surface density and then remained constant for values above 5 Mo/nm(2). These effects appear to reflect the tendency of Al-O-Mo species to adsorb alkoxide intermediates and favor their sequential oxidation to COx. Propene combustion rate constants also decreased relative to those for propane ODH as two-dimensional structures form with increasing Mo surface density.