The hydrogenation of CO and CO2 over Rh/SiO2 have been investigated for the purpose of identifying the similarities and differences between these two reaction systems. In situ infrared spectroscopy was used to characterize the surface of the catalyst. Exposure of the catalyst to CO or CO2 produced very similar infrared spectra in which the principal features are those for linearly and bridge-bonded CO. In the case of CO2 adsorption, a band for weakly adsorbed CO2 could also be observed. For identical reaction conditions the rate of CO2 hydrogenation to methane is higher than that for CO hydrogenation. The activation energy for CO hydrogenation is 23.2 kcal/mol and that for CO2 hydrogenation is 16.6 kcal/mol. The partial pressure dependances on H-2 and COz (z = 1, 2) are 0.67 and -0.80, respectively, for CO hydrogenation, and 0.53 and -0.46, respectively, for CO2 hydrogenation. Infrared spectroscopy reveals that under reaction conditions the catalyst surface is nearly saturated by adsorbed CO. The spectra observed during CO and CO2 hydrogenation are similar, the principal difference being that the CO coverage during CO hydrogenation is somewhat higher than that during CO2 hydrogenation. The CO coverage is insensitive to Hz partial pressure, but increases slightly with increasing COz partial pressure. Transient-response experiments demonstrate that the adsorbed CO is a critical intermediate in both reaction systems. It is proposed that the rate determining step in the formation of methane is the dissociation of H2CO, produced by the stepwise hydrogenation of adsorbed CO. A rate expression derived from the proposed mechanism properly describes the experimentally observed reaction kinetics both under steady-state and transient-response conditions.