Silica-supported molybdenum oxide and vanadium oxide catalysts have been prepared with a wide range of surface metal loadings (0.3-3.5 metal atom/nm(2)), Isolated surface metal oxide species are dominant at surface metal loadings on silica below ca. 1 metal atom/nm(2) while at higher loadings, the metal oxide species tend to aggregate into crystalline MoO3 and V2O5 particles. Both catalyst series have been tested in the selective oxidation of methane with molecular oxygen at atmospheric pressure. The reactivity for methane conversion appears to be essentially related to dispersed isolated surface metal oxide species, with the surface vanadium oxide species being more reactive than molybdenum oxide species. The higher reactivity of the surface vanadium oxide sites may be related to the different nature of the interaction of the oxides with oxygen, Silica-supported vanadium oxide catalysts, despite their higher activity, result in lower yields to HCHO than silica-supported molybdenum oxide catalysts.