A series of MoO3/SiO2 samples prepared by several research groups, employing different silicas and preparation methods, have been studied by in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and methanol oxidation as a probe reaction. The in situ Raman spectroscopy studies show that under dehydrated conditions the molecular structure of the silica-supported molybdenum oxide surface species is independent of the preparation methods used in the present study. The surface molybdenum oxide species is assigned to an isolated, highly distorted octahedral mono-oxo Mo structure. The appearance of new structures in some samples is due to the presence of calcium impurities in the silica supports which result in the formation of calcium molybdate. Neither the preparation method nor the specific silica used affected the methanol oxidation activity of the MoO3/SiO2 catalysts. The surface molybdenum oxide coverage on silica is the only relevant factor that determines the catalytic properties during methanol oxidation. In siu Raman spectroscopy during methanol oxidation shows aggregation of surface molybdenum oxide species to crystalline beta-MoO3. The extent of aggregation increases with the surface molybdenum coverage, even at very low coverages, and accounts for the decrease in the methanol oxidation catalytic activity with increasing surface molybdenum coverage.