The catalytic behaviour of SiO2, MoO3/SiO2, V2O5/SiO2, TiO2, and V2O5/TiO2 systems in the partial oxidation of methane to formaldehyde with O-2 (MPO) has been systematically evaluated by temperature programmed reaction (TPR) measurements in the range 400-800 degrees C. The effects of MoO3 (2-7 wt%) and V2O5 (2-20 wt%) loading on the surface reactivity of the SiO2 support have been assessed. A sequential reaction path (CH4 --> HCHO --> CO --> CO2) accounts for the formation of oxygenated products on all SiO2 based oxide catalysts at T<650 degrees C, while a surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C-2 products both on SiO2 and MoO3/SiO2 catalysts at T greater than or equal to 700 degrees C. MoO3 depresses the specific surface activity (SSA, nmol(CH4), s(-1) m(-2)) of the bare silica at T<650 degrees C, while V2O5 acts as a promoter of the SSA of both SiO2 and TiO2 carriers at any T. The maximum SSA on medium loaded (5-10 wt%) V2O5/SiO2 catalysts has been observed though HCHO selectivity steadily decreases with V2O5 loading. The marked redox behaviour of TiO2 based catalysts enables the prevailing formation of COx at any T. The different reactivity of SiO2 and TiO2 supports as well as their influence on the catalytic performance of supported oxide systems have been discussed.