The partial oxidation of methane to formaldehyde (MPO) with molecular O-2 has been investigated on various bare and Fe-doped commercial silica catalysts at 650degreesC. The influence of the Fe loading (0.01-3.2 wt.%) on the catalytic pattern and steady-state properties of the catalysts have been evaluated. The dispersion and coordination symmetry of surface Fe3+ species have been investigated by EPR analysis. "Isolated Fe3+ ions", "small Fe2O3 clusters" and "Fe2O3 particles" are present on the surface of the FeOx/SiO2 catalysts, their relative concentration depending upon the loading. Iron species, irrespective of the state of aggregation, enable the reactivity of the silica surface, whilst the highest HCHO productivity is associated with an optimum surface Fe loading (0.05-0.1 Fe-at nm(-2)). The different reactivity of the various Fe-doped silica catalysts has been rationalised and normalised in terms of different concentration of "isolated Fe3+ species" and "aggregated Fe2O3 moieities".