A novel high-capacity, water-tolerant adsorbent for CO2 was developed. It consisted of diethanolamine (DEA) loaded pore-expanded MCM-41 silica (PE-MCM-41). Due to its very large pore volume, PE-MCM-41 silica was capable of accommodating a greater quantity of amine resulting in higher CO2 adsorption capacity compared to the other supports including activated carbon, silica gel, and standard MCM-41 silica. Adsorption measurements were conducted by gravimetry using dry CO2 to obtain uptake curves and apparent rate data. The capacity and uptake rate reached maxima with respect to amine content and then declined due to the deposition of excess amine on the particle's external surface and within the interparticle voids. At CO2 partial pressures below 0.15 atm, the current DEA loaded PE-MCM-41 adsorbent was found to be superior to the more conventional zeolite 13X. Adsorption studies with humid CO2 revealed that the adsorption capacity of the PE-MCM-41-based material was insensitive to the presence of moisture, which represents a major advantage over zeolite 13X. Repeated adsorption-desorption cycles revealed that our novel adsorbent exhibited good cyclic stability.