Substantial efforts have been made to increase the CO2 working capacity of amine adsorbents for an efficient CO2 capture. However, the more important metric for assessing adsorbents is the regeneration heat required for capturing a fixed amount of CO2. In this work, we synthesized polyethyleneimine (PEI)/SiO, adsorbents functionalized with various epoxides. This provided adsorbents with six different amine structures showing various CO2/H2O adsorption properties. Our studies revealed that the CO2 working capacity was not a decisive factor in determining the regeneration heat required for CO2 capture. This is because the benefit of large CO2 working capacity was canceled out by the difficulty of CO2 desorption. Instead, the suppression of H2O co-adsorption was critical for reducing the regeneration heat because substantial latent heat is required for H2O desorption. Consequently, the PEI/SiO2 functionalized with 1,2epoxybutane required a much lower regeneration heat (2.66 GJ tCO(2)1) than the conventional PEI/SiO2 (4.03 GJ tCO(2)-1) because of suppressed H2O co-adsorption as well as moderately high CO2 working capacity.