The interaction of CO with ceria under conditions typically used to measure the oxygen storage capacity (OSC) of automotive three way catalysts (TWC) has been investigated by in situ Raman spectroscopy. During exposure of the ceria to CO at 623 K vibrational bands at 1582-1600 and 1331-1340 cm(-1) appeared; these bands increased with increasing time of exposure to CO. The band positions are consistent with phonon modes of carbon; however, assignment to carboxylate species or carbonate species cannot be excluded. Subsequent exposure to O-2 at room temperature resulted in a decrease in the intensities of the 1582-1600 and 1331-1340 cm(-1) bands by more than 90%. As well, exposure to O-2 at room temperature also resulted in the appearance of Raman modes characteristic of formate and peroxide surface species. The mechanism by which formate forms upon room temperature O-2 exposure is discussed in the context of the assignment of the 1582-1600 and 1331-1340 cm(-1) bands to carbon phonon modes which result from the disproportionation of CO on reduced ceria.