The nature and structure of adsorbed CO2 on immobilized amine sorbent in the presence and absence of H2O vapor have been studied by in situ infrared spectroscopy. CO2 adsorbed on the primary amine as ammonium carbamate and on the secondary amine as carbamic acid. Adsorbed H2O mainly on secondary amine enhanced CO2 capture capacity by increasing accessibility of amine sites and promoting the formation of carbamic acid. The binding strength of the adsorbed species increased in the order: carbamic acid < adsorbed H2O < paired carbamic acid; ammonium carbamate < ammonium chloride. Flowing argon over the amine sorbent at 50 degrees C removed weakly adsorbed H2O and carbamic acid from the secondary amine sites. Raising temperature is required to completely regenerate sorbent by removing strongly adsorbed ammonium carbamate from the primary amine sites and paired carbamic acid. The results of this study clarify the role of H2O vapor in amine-sorbents for CO2 capture and provide a molecular basis for the design of the sorbents and operation of amine-based CO2 capture processes.