Samples of molybdenum oxide supported on SiO2, Al2O3, and TiO2 were used to study the effect of loading and support on propylene oxidation. The samples were characterized by oxygen chemisorption, temperature programmed reduction (TPR), and temperature programmed surface reaction (TPSR) of adsorbed ethanol. Oxygen chemisorption and TPR results indicated that the molybdenum oxide-support interaction increased in the order SiO2 < Al2O3 < TiO2. TPSR of adsorbed ethanol was used to characterize the acid-base properties of the catalysts. These properties played an important role in the surface reactions of the highly dispersed low-loading samples and resulted in the formation of oxidation products depending on the supports (acrolein on MoO3/SiO2, acetaldehyde on MoO3/ Al-O3, and acetone on MoO3/TiO2). In higher loading samples these differences were reduced as support effects became less important.