The partial oxidation of methane to formaldehyde with molecular O2 has been investigated on various commercial bare SiO2 samples and silica-supported MoO3 and V2O5 catalysts at 550-650-degrees-C in batch, pulse, and continuous flow reactors at 1.7 bar. Amongst the different SiO2 samples, the highest HCHO productivity expressed as space time yield (STY(HCHO), g . kg(cat)-1 . h-1) is found with "precipitated" silica, while "fumed" SiO2 results in the least reactive silica. Incorporation of molybdena depresses the STY(HCHO) value for the "precipitated" silica but enhances the STY(HCHO) for bare "fumed" silica. In contrast, addition of vanadia to either "precipitated" or "fumed" silicas leads to higher STY(HCHO) values. On the basis of a series of experiments performed by continuous scanning of the reaction mixture with a quadrupole M.S., the participation of lattice oxygen in the main reaction pathway has been ruled out. The acidic properties of the catalysts have been compared by ZPC and NH3-TPD measurements. A straight correlation between the density of reduced sites (rho, 10(16) s(r) . g(cat)-1), evaluated in steady-state conditions by O2 chemisorption, and the reaction rate has been disclosed. MoO3 and V2O5 dopants modify the catalytic properties Of SiO2 by affecting the process of oxygen activation on the catalyst surface.