Diffuse-reflectance IR Fourier transform (DRIFT) spectroscopy was used to study in situ, the low-temp. (T <200 Deg) methanation of CO2 over Ru on TiO2 and on Al2O3 supports. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 +- 0.05 order in H2 concn., and essentially independent of CO2 concn. At 110 Deg, 40% of the available metal sites are occupied by CO (qCO = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and qCO = 0.2, is 15 times less active. Batch catalyst screening expts. showed no dependence of methanation activity on adsorbed CO(COa) formation rate (as modeled by HCOOH dehydration) or on qCO. In view of this, and the fact that CO dissocn. is structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOOa- intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixts. of Ru/anatase and Ru/rutile, the former being a better catalyst for COa supply while the latter is more effective in COa hydrogenation. [on SciFinder (R)]