DFT and high-level ab initio calculations (among them B3LYP and G3MP2B3) have been used to describe molecular reactions relevant for CO2 absorption in aqueous (alkanol)amine solutions. Reaction mechanisms for various reactions of CO2 with ammonia, monoethanolamine (MEA), and diethanolamine (DEA) to carbamic acid and ion pair products have been investigated and interpreted in light of experimental observations. Additional water, ammonia, MEA, and DEA molecules have also been added to the molecular complexes to simulate microsolvation effects. These extra molecules may act as catalysts for the desired reactions, and in several cases they have a large impact on activation and reaction energies. Solvent effects were estimated by applying electrostatic continuum models for selected systems. Our calculated transition state energies agree well with experimental activation energies.