Ethanol oxidation over supported molybdenum oxide was studied by in situ laser Raman spectroscopy, deuterium isotopic substitution, and steady-state kinetic measurements. Isotopic substitution experiments established that the same mechanism occurred over 9% MoO3/SiO2, 15% MoO3/Al2O3, and 9% MoO3/TiO2. The reaction was found to take place in two stages, an equilibrated adsorption step to form adsorbed ethoxide species, and either one of two rate-determining steps : a-hydrogen abstraction to form acetaldehyde or beta-hydrogen abstraction to form ethylene. Laser Raman spectroscopy was used to directly measure the concentration of the surface ethoxide intermediates during reaction. On the SiO2-supported catalysts two types of adsorbed ethoxide species were observed. One, associated with Mo=O bonds, was a true reactive intermediate, while the other, associated with Mo-O-Mo bonds, was an unreactive spectator on the surface. Analysis of the reaction isotherms showed that activation energy decreased with increasing coverage and could be fit to a Frumkin-Temkin equation theta = (1/f) ln(1 + (K1PC2H5OH)-P-0), characteristic of a nonuniform surface. Consistent with this, the kinetics could be described by Temkin’s theory of rates on nonuniform surfaces, with a rate constant of the form k(2) = k(2)(0)exp[(1 - alpha)f theta]. In these equations theta is the coverage, f is the breath of the nonuniformity (f = 11), and a is a Bronsted parameter (alpha = 0.14).