Mesoporous silica substrates were functionalized with N-[3-(trimethoxysilyl)propyl]diethylenetriamine to form diethylenetriamine[propyl(silyl)]- (DT-) functionalized hybrid products suitable for CO2 adsorption. The materials prepared were characterized by N-2 adsorption/desorption at 77 K, C and N elemental analysis, helium pycnometry, X-ray diffraction (XRD), CO2 adsorption, and thermal decomposition and were compared to analogous aminopropylsilyl- (AP-) and ethylenediamine[propyl(silyl)]- (ED-) functionalized materials. The extent of surface functionalization varied with substrate morphology. CO2 adsorption capacities and heats of adsorption were determined via combined thermogravimetric analysis and differential thermal analysis (TGA/DTA). Functionalization of the substrates was found to enhance their CO2 adsorption capacities at 20 degrees C under anhydrous conditions. Higher temperature led to reduced adsorption capacities but higher heats of adsorption (H-ads) of CO2, thought to be due to the reduced role of weak physisorption sites. When CO2 was supplied in a moist gas stream, the adsorption capacity was reduced, but the value of H-ads(CO2) was essentially unchanged. The thermal stabilities of one substrate and its AP-, ED-, and DT-functionalized products both in N-2 and in mildly oxygenated N-2 were also characterized by combined TGA/DTA. These materials were found to be stable up to 170 degrees C in both atmospheres and, furthermore, had no particular affinity for either N-2 or O-2 over this temperature range. Oxidative decomposition data from TGA at higher temperature were found useful for estimating the N content of these materials.