The oxidation of ethanol on MoO3 supported on SiO2, Al2O3, and TiO2 was studied in a flow reactor at atmospheric pressure. The reactivity sequence followed the order MoO3/TiO2 > MoO3/Al2O3 > MoO3/SiO2 and correlated with the reducibility of the surface molybdenum species. Ethanol oxidation produced mainly acetaldehyde, diethyl ether, and ethylene through ethoxide type intermediates adsorbed on different sites (M=O, Mo-O-Mo, or Mo-O-M), Two types of ethoxide species were identified using laser Raman spectroscopy under in situ conditions and could be associated with the Mo=O and Mo-O-Mo sites. Although rates were strongly affected by the support, the rates did not depend substantially on the loading, This was due to a compensation between the equilibrated adsorption of ethanol and the rate-determining decomposition of the ethoxide intermediates. The activation energy did not vary with support, suggesting that activity was controlled by a term in the preexponential factor. The link to reducibility and the existence of a common ethoxide intermediate indicated that the controlling factor was likely to be the electronic partition function associated with the density of electron-accepting levels in the molybdate-support complex.