Transient experiments were applied to the study of the adsorbed CO intermediate, (CO)(a), formed during the CO2 methanation reaction on a 2% RU/TiO2 catalyst at 383 K. Step-up experiments showed that the (CO)(a) formation steps are inhibited by H2O and enhanced by H-2. Step-down experiments showed that the (CO)(a) hydrogenation is not influenced by the partial pressure of water. Based on the fact that water inhibits the overall CO2 methanation, it is deduced that the rate limiting process in the overall reaction is (CO)(a) formation.