This work examines the stability of polyethylenimine(PEI)-impregnated mesoporous silica for CO2 removal over a wide range of conditions. The support used was a SBA-15 silica with platelet morphology and short pore channels (SBA-15PL). The effect of long-term exposure to different gaseous streams, including carbon-free air (CFair), simulated flue gas (SFG), and different CO2/O-2/N-2 mixtures on the carbon dioxide adsorption capacity was investigated. Extensive CO2 adsorption desorption cycling using dry and humid streams at different adsorption and regeneration temperatures was also carried out. Based on adsorption data, as well as diffuse reflectance infrared Fourier transform (DRIFT) and C-13 CP MAS NMR measurements, it was found that PEI-modified adsorbents exhibit (i) high thermal stability at moderate temperatures, (ii) highly stable CO, uptake in the presence of moisture, (iii) extensive degradation in the presence of dry CO2, particularly at high temperature, (iv) fast degradation upon exposure to CFair even at moderate temperatures, (v) excellent stability in the presence of humidified gases containing both CO2 and O-2.