Carbon dioxide adsorption was investigated in the presence of polyethylenimine (PEI)-impregnated mesoporous silicas with different pore lengths, namely, pore-expanded MCM-41, conventional SBA-15 with different pore diameters (7.2 and 10.5 nm), and SBA-15 with platelet morphology. The pore lengths of the silica supports were ca. 25, 1.5, and 0.2 mu m, respectively. Under comparable conditions, the adsorption performance was found to be strongly dependent upon the pore length. The materials with the shortest channels showed the highest capacity and fastest adsorption. These findings were associated with diminished diffusion resistance and enhanced amine accessibility inside the pores.