Four different catalysts (Ni/SiO2, Ni/Al2O3, Ru/SiO2, Ru/Al2O3) were extensively investigated in the TAP reactor, in order to elucidate the role of the metal and support in the dry reforming of methane. Over nickel, dissociative CH4 adsorption leads to H-2 gas and to an accumulation of carbon species on the surface. Dissociative CO2 adsorption lakes place giving CO gas and adsorbed oxygen. The latter reacts with carbon, originating from CH4, on the surface to give a second CO molecule in the rate determining step. Over ruthenium, the methane activation is the slowest step and CO2 reacts directly with adsorbed carbon to CO. The surface oxygen accumulation is minimal and therefore, the oxidation hydrogen is suppressed. Whereas, the silica support is rather inert, the alumina support participates in the reaction by delivering oxygen atoms to the metal via hydroxyl groups spill-over. In addition, a strong interaction between gaseous CO/CO2 and oxygenated adspecies is observed on alumina. The reverse water gas shift reaction takes place under the TAP conditions. The different results are discussed in terms of the hydrogen selectivity.