The objective of the work presented here is to develop a nanoporous solid adsorbent which can serve as a "molecular basket" for CO2 in the condensed form. Polyethylenimine (PEI)-modified mesoporous molecular sieve of MCM-41 type (MCM-41-PEI) has been prepared and tested as a CO2 adsorbent. The physical properties of the adsorbents were characterized by X-ray powder diffraction (XRD), N-2 adsorption/desorption, and thermogravimetric analysis (TGA). The characterizations indicated that the structure of the MCM-41 was preserved after loading the PEI, and the PEI was uniformly dispersed into the channels of the molecular sieve. The CO2 adsorption/desorption performance was tested in a flow system using a microbalance to track the weight change. The mesoporous molecular sieve had a synergetic effect on the adsorption of CO2 by PEI. A CO2 adsorption capacity as high as 215 mg-CO2/g-PEI was obtained with MCM-41-PEI-50 at 75 degreesC, which is 24 times higher than that of the MCM-41 and is even 2 times that of the pure PEI. With an increase in the CO2 concentration in the CO2/N-2 gas mixture, the CO2 adsorption capacity increased. The cyclic adsorption/desorption operation indicated that the performance of the adsorbent was stable.