Adsorption of CO2 to uranium oxide, (UO3)(n), clusters was modeled using density functional theory (DFT) and coupled cluster theory (CCSD(T)). Geometries and reaction energies were predicted for carbonate formation (chemisorption) and Lewis acid-base addition of CO2 (physisorption) to these (UO3)(n) clusters. Chemisorption of multiple CO2 moieties was also modeled for dimer and trimer clusters. Physisorption and chemisorption were both predicted to be thermodynamically allowed for (UO3)(n) clusters, with chemisorption being more thermodynamically favorable than physisorption. The most energetically favored (UO3)(3)(CO2)(m), clusters contain tridentate carbonates, which is consistent with solid-state and solution structures for uranyl carbonates. The calculations show that CO2 exposure is likely to convert (UO3)(n) to uranyl carbbnates.