The possibility of CO2 as an alternative oxidant for the direct conversion of CH4 to formaldehyde was investigated. The activity of the catalyst was measured at an atmospheric pressure and temperatures at which the homogeneous gas-phase reactions were negligible. Among various metal oxides loaded on SiO2 catalysts, only vanadium oxide produced the desired product. V2O5/SiO2 and V2O5/oxidized diamond catalysts, the most effective catalysts for formaldehyde synthesis, afforded about 500 mumol h(-1) g-cat(-1) and 300 mumol h(-1) g-cat(-1) of formaldehyde at 973 K, respectively. When the reaction of CH4 was carried out in Ar atmosphere, both of the V2O5/SiO2 and V2O5/oxidized diamond catalysts lost the catalytic activity as soon as the lattice oxygen of vanadium oxide was consumed. However, the activity recovered by switching the atmosphere from Ar to CO2. These results strongly suggest that CO2 acts as an oxidant for selective oxidation of CH4 to formaldehyde via the lattice oxygen of vanadium oxide. (C) 2003 Elsevier B.V. All rights reserved.