The methanation of carbon dioxide has been studied over 3% Ru/Al2O3 and 20% Ni/Al2O3 commercial catalysts. Experiments have been performed in diluted conditions in a flow catalytic reactor with a continuous IR detection of products. The data, reported here, confirm that 3% Ru/Al2O3 is an excellent catalyst for CO2 methanation (96% methane yield with no CO coproduction at 573 K at 15,000 h−1 GHSV in excess hydrogen). The performance is better than that of Ni/Al2O3 catalyst. The reaction orders over both catalysts with respect to both hydrogen and CO2 were determined over conditioned catalysts. A conditioning of the Ru/Al2O3 catalyst by reactant gas stream was found to be needed and more effective than conditioning in hydrogen, possibly because water vapour formed during methanation reaction will react to remove chlorine impurities from catalyst surface Conditioned Ru/Al2O3 catalyst was found to retain stable high activity after different shut-down and start-up procedures, thus being possibly applicable in intermittent conditions.