An extensive correlated molecular orbital theory study of the reactions of CO 2 with a range of substituted amines and H 2 O in the gas phase and aqueous solution was performed at the G3(MP2) level with a self-consistent reaction field approach. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH 3 reactions. A catalytic NH 3 reduces the energy barrier more than a catalytic H 2 O for the formation of H 2 NCOOH and H 2 CO 3 . In aqueous solution, the barriers to form both H 2 NCOOH and H 2 CO 3 are reduced, with HCO 3 - formation possible with one amine present and H 2 NCOO - formation possible only with two amines. Further reactions of H 2 NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH 3 NH 2 , CH 3 CH 2 NH 2 , (CH 3 ) 2 NH, CH 3 CH 2 CH 2 NH 2 , (CH 3 ) 3 N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (Δ G ‡ aq < 15 kcal/mol) and alkylammonium bicarbonate (Δ G ‡ aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO 2 capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.