Reactions of laser-ablated lanthanoid atoms (except for radioactive Pm) with carbon dioxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. On the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments, the lanthanoid oxocarbonyl complexes have been identified. Density functional theory calculations have been performed on these products, which support the experimental assignments of the infrared spectra. Infrared spectroscopic studies of these lanthanoid complexes combined with theoretical calculations reveal that the early lanthanoid (La-Sm) oxocarbonyl complexes adopt trans configurations, the europium and ytterbium ones adopt side-on-bonded modes (Eu-(eta2-OC)O and Yb-(eta2-OC)O), and the late lanthanoid (Gd-Lu) ones adopt cis configurations. Natural bond orbital analysis indicates that the formation of the lanthanoid oxocarbonyl complexes involves the promotion of 6s and 4f electrons into the metal valence shell.