CO insertion is an important reaction step in the fields of both heterogeneous and homogeneous catalysis for the production of oxygenated fuels and useful C-2+ oxygenates. Enhancement of CO insertion rate on the supported Rh catalysts by the addition of oxophilic promoters (i.e., Ce, Mn, Fe, and La oxide) has been suggested to result from the high activity of a postulated tilted CO of which oxygen interacts with the oxophilic promoter. Investigation of the reactivity of adsorbed CO by in situ infrared spectroscopy combined with transient studies reveals that tilted CO on Ce-Rh/SiO2 is inactive for CO insertion at 303 K. The reactivities of CO species toward CO insertion were determined to be as follows : Rh(CO)(4) > linear CO > gem-dicarbonyl CO. Steady-state isotopic transient studies show that gaseous CO rapidly exchanges with adsorbed CO and Ce decreases the residence time of intermediates leading to propionaldehyde formation.