Oxidative dehydrogenation (ODH) of propane was studied on zirconia-supported molybdenum oxide catalysts. The structure of the ZrO2 support and of the dispersed MoOx species was characterized by X-ray diffraction and by Raman and UV-visible spectroscopies. The structure of dispersed molybdena depends on the Mo surface density and on the temperature at which catalyst precursors are treated in air. Polymolybdate domains were detected by Raman at Mo surface densities below 5 Mo/nm(2). At higher surface densities, MoO3 and ZrMo2O8 are present; their relative concentrations depend on the pretreatment temperature. Below 773 K, MoO3 is the predominant structure at high surface densities, but ZrMo2O8 forms above 773 K. UV-visible edge energies decrease with increasing surface density for samples containing polymolybdate species, suggesting that MoOx domains become larger as the Mo surface density increases. ODH turnover rates decrease with increasing Mo surface density on samples containing polymolybdate species and MoO3. This trend is accompanied by an increase in the initial propene selectivity and in the vibrational frequency of Mo=O bonds. Higher Mo=O vibrational frequencies reflect stronger Mo=O bonds, which show lower ODH reactivity; therefore, the lower ODH reaction rates (per Mo atom) at higher Mo surface densities arise from the lower reactivity of Mo=O bonds, while higher initial propene selectivities arise either from the decrease of exposed Mo-O-Zr bonds or the lower reactivity of Mo=O bonds as the size of MoOx domains increases with increasing Mo surface density. At similar Mo surface densities, samples containing predominantly ZrMo2O8/ZrO2 show higher turnover rates and lower initial propene selectivities than those containing MoO3 species because the vibrational frequency of the Mo=O bond for ZrMo2O8/ZrO2 is lower than that for MoO3. ODH turnover rates over ZrMo2O8/ZrO2 also decreased with increasing Mo surface density, ultimately due to the increase of the particle size which leads to lower propane accessibility. (C) 2000 Academic Press.