Laser-ablated U and Th atoms react with CO2 to give OUCO (1806.9, 823.2 cm(-1)) and OThCO (1778.4 cm(-1)) as major products, which are identified by isotopic substitution ((CO2)-C-13, (CO2)-O-18 and (CO2)-C-12 + (CO2)-C-13, (COO)-O-16-O-18 + (CO2)-O-18 mixtures), doping with CCl4 as an electron trap, and comparison with frequencies from density functional calculations. The insertion reaction is spontaneous as these bands increase on annealing the neon matrix from 4 to 6 K. Further reaction of OAnCO (An = Th, U) with another CO2 molecule gives the O(2)An(CO)(2) product, although much more so with uranium than thorium. Reaction of the metal cations with CO2 produces OUCO+ (2073.0, 881.2 cm(-1)) and OThCO+ (2009.9, 896.4 cm(-1)) with higher yield in thorium experiments. The O(2)An(CO)(2)(-) anions are observed for both metals again with higher yield in uranium investigations.