Transient expts. were applied to the study of the adsorbed CO intermediate, (CO)a, formed during CO2 methanation on 2 wt.% Ru/TiO2 catalyst at 383 K. Step-up expts. showed that the (CO)a formation steps are inhibited by H2O and enhanced by H2. Step-down expts. 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 was deduced that the rate limiting process in the overall reaction is (CO)a formation. [on SciFinder (R)]