Aminosilica materials are promising candidates for CO2 capture from dilute streams such as ambient air and flue gas. Most aminosilica sorbents are constructed using primary and/or secondary amines, which have been shown to primarily react with CO2 to form alkylammonium carbamates and related structures. While ammonium bicarbonate formation is known to occur in aqueous amine solutions in the presence of CO2, there has been conflicting evidence of its formation on solid supported analogues. To probe if the ammonium bicarbonate species can exist on solid supported amines, tertiary amines, which are known to form bicarbonates in aqueous solution, are grafted onto mesoporous silica SBA-15, and the materials are further characterized using in situ FTIR spectroscopy and solid-state NMR spectroscopy in the presence of humid and dry CO2. Dry and humid CO2 capacities for these sorbents are also evaluated using fixed bed experiments and thermogravimetric analysis. This work shows that ammonium bicarbonates can exist on solid supported amines but also demonstrates that tertiary amines are poor CO2 sorbents under the conditions employed.