Selectivities in methanol oxidation over silica supported molybdenum oxide catalysts were investigated in relation with the phase distribution. The supported catalysts were prepared by impregnation with ammonium heptamolybdate. In addition to crystalline MoO3, Mo containing cluster species of 1-2 nm size were observed by STEM even from a used catalyst with 13% catalyst loading. The percentage of Mo present as crystalline MoO3 increases with the catalyst loading. An ESCA study indicates that part of surface Mo in the supported catalysts is reduced to Mo5+. The dimethyl ether selectivity increases with the catalyst loading and its formation occurs over the crystalline MoO3 phase. The selectivities to CO and methyl formate are greatly enhanced because of the presence of support, and are relatively independent of the catalyst loading and phase distribution. The dependence and independence of the selectivities of different byproducts on the loading make the silica supported catalysts with high catalyst loadings less selective for the partial oxidation of methanol to formaldehyde.