Flue gases from coal-fired power plants typically contain not only CO2 but other acid-gas impurities such as SOx and NOx that can dramatically influence the CO2 capture efficiency. Whereas postcombustion CO2 capture by aminosilica materials has been extensively studied over the past few years because of their high equilibrium CO2 capacities, the performance of these materials under realistic conditions (in the presence of SOx, NOx, and O-2) remains relatively unexplored. In this study, the degree of irreversible binding of SO2, NO, and NO2 to four supported amine adsorbents is evaluated to assess the SO2, NO, and NO2 adsorption capacities of aminosilica sorbents and their effects on the CO2 adsorption capacities. Adsorbents constructed using poly(ethyleneimine) and three different silane coupling agents (based on propyltrimethoxysilane linkers) with primary, secondary, and tertiary amines are evaluated. Under the experimental conditions used in this investigation, it is found that primary amines with high amine loadings displayed more affinity toward NO than their secondary and tertiary amine counterparts. However, overall, NO adsorption on the aminosilica adsorbents is low, and therefore, the CO2 capacities of the adsorbent materials exposed to NO remained almost unchanged after the exposure. In contrast, all materials showed a very high nitrogen dioxide adsorption capacity upon exposure to NO2. As a result, all adsorbents treated with NO2 exhibited a dramatic reduction in CO2 capacity, which corresponds to the deactivation of amine groups due to the irreversible binding of NO2. In addition, our results indicate that SO2 adsorbed significantly on supported amine adsorbents, resulting in a dramatic loss in CO2 capacity during CO2 capture from flue gas. With similar amine loadings, although secondary amines exhibited higher affinity to SO2, their CO2 capacity loss after exposure to SO2 is lower than that of primary amines, indicating that these materials are more stable in the presence of SO2, which implies that more SO2 desorbs from secondary amines during the desorption step. These results suggest that for silica-supported amine materials to be useful in practical CO2 capture applications, it is necessary to significantly reduce the SO2 and NO2 concentrations of the flue gas prior to the CO2 capture process. On the other hand, the capture efficiency of these materials does not change significantly in the presence of NO. This suggests that such materials might be promising for postcombustion CO2 capture from flue gas streams derived from natural gas combustion, as these streams typically contain reduced SO2 concentrations but can still have significant NOx concentrations.