Conventional MCM-41 and pore-expanded MCM-41 (PE-MCM-41) silicas have been used as supports for grafting 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane (TRI) and tested for CO2 adsorption. The effects of the quantity of triamine silane added to the grafting mixture on the CO2 adsorption capacity and apparent adsorption rate have been examined. The results showed that when both supports were grafted under the same conditions, PE-MCM-41 was grafted with slightly larger quantities of amine than MCM-41, for all controlled silane additions. Based on the adsorption performance of the materials using a dry 5% CO2/N-2 feed mixture, the optimal quantity of triamine silane added to the grafting mixture was determined to be ca. 3.0 cm(3)/g(SiO2), for both MCM-41 and PE-MCM-41. The CO2 adsorption capacity of TRI-PE-MCM-41 was significantly higher than that of TRI-MCM-41. Furthermore, the dynamic adsorption performance of TRI-PE-MCM-41 was far superior to TRI-MCM-41. In comparison to 13X zeolite, TRI-PE-MCM-41 exhibited higher adsorption capacities in the initial time frame of exposure, even though the 13X zeolite exhibited a higher equilibrium adsorption capacity. The result of this behavior is largely due to the rapid CO2-amine interaction and the open pore structure of TRI-PE-MCM-41 over that of the 13X zeolite. When these adsorbents were exposed to a humid stream of 5% CO2/N-2 (28% relative humidity), both grafted materials exhibited a slight increase in the adsorption capacity, whereas, 13X zeolite did not retain any significant CO2 adsorption capacity. These results suggest that the TRI-PE-MCM-41 material may be most suitable for use in a rapid cyclic adsorption process under humid feed conditions.