The nature and characteristics of the catalytic surface of supported MoO 3 catalysts were studied. Changes that occurred on oxidizing alcohols in air over carbon and silica supported MoO 3 were examined. Structural data were combined with electron microscopy and photoelectron spectroscopy to demonstrate that the carbon support promotes segregation and fragmentation of MoO 3 , whereas sintering occurs on silica. Results indicated that this may be correlated with a synergism between the carbonaceous material and metal oxide which provides a reoxidation pathway for reduced Mo, thus preventing formation of extended zones of MoO 2 , which is inactive for the oxidation of alcohols.