Adsorption of CO, used as infrared molecule probe, was performed at room temperature or at low temperature (140 K), on either chlorine-free or chlorine-containing ceria, in order to investigate both the chlorine effect and the ceria reduction state. On ceria and Pd/ceria catalysts, chlorine acts as an electronegative ligand. Its effect extends to the raft-like structure of palladium in Pd/CeO2 catalysts. Moreover, ceria chlorination prevents palladium from CO clustering. CO adsorption on the chlorine-containing Pd/CeO2 catalyst led to some water formation, tentatively assigned to an incomplete reduction of microporous ceria sites. Moreover, oxalate-like species were evidenced and considered as impurities on residual reduced carbonate sites.