The mechanism of bulk tungsten carbide catalysts synthesis from WO3 in CH4/H-2 mixtures has been studied using temperature programmed reactions associated with CH4/D-2 exchange reaction and in situ X ray diffraction, Various experimental parameters have been studied such as partial pressures of reactants, heating rate, mass of precursor, or flow rate in order to determine the most important steps occurring during the transformation of WO3 to WC. It is shown that at temperatures below 900-923 K the diffusion within the solid particles is slow with respect to the rate of reduction of the surface, allowing the carburization of the surface in the presence of a core still partially oxidized. At higher temperatures, the diffusion is rapid, leading to a uniform reduction within the solid. In this case the surface is continuously replenished in oxygen thus inhibiting the activation of methane and allowing the carburization to proceed only when the solid is deeply reduced. An inhibiting effect of hydrogen pressure on the interaction of methane with the surface has also been evidenced, an effect which excludes the possibility of an independent control of the reduction process from that of carburization, Finally the role of space velocity has also been elucidated. (C) 2000 Academic Press.