The characteristics and CO2 adsorption of mesoporous silicas including MCM-41, SBA-15, and pore-expanded MCM-41 with pore size in a range of 2-17 nm, modified by mono-, di-, and tri-aminosilanes were examined in this study. The adsorption data show that SBA-15 possessing a proper pore size accommodated amines exhibit the best CO2 adsorption capacity as 2.41 mmol CO2/g at 333 K. The corresponded CO2 adsorption efficiencies, represented by CO2/N molar ratios, were as high as of 0.5-0.7, an indication of the adsorption being sufficiently close to thermodynamic equilibrium. The CO2 adsorption mechanism of the amine-functionalized mesoporous silica is proposed. The adsorption of the grafted amine under anhydrous CO2 can be considered to proceed through carbamate intermediate and zwitterion mechanism. (C) 2009 Elsevier B.V. All rights reserved.