Transient measurement techniques were applied to the kinetic investigation of the reaction mechanism of CO2 methanation, using diffuse-reflectance IR spectroscopy and mass spectrometry. The coupled information of the surface intermediates and the time evolution of the gas-phase components leads to accurate identification of spectator species on the surface. Reaction intermediates (e.g., CO and formates) were identified. The former is a key intermediate, and its hydrogenation leads to methane formation. The latter is fixed on the support, in equil. with a active formate species on the interface metal-support. A reaction mechanism was proposed, including the formation of formates through a carbonate species. [on SciFinder (R)]