A fundamental study on the adsorption properties of primary, secondary, and tertiary amine materials is used to evaluate what amine type(s) are best suited for ultradilute CO2 capture applications. A series of comparable materials comprised of primary, secondary, or tertiary amines ligated to a mesoporous silica support via a propyl linker are used to systematically assess the role of amine type. Both CO2 and water adsorption isotherms are presented for these materials in the range relevant to CO2 capture from ambient air and it is demonstrated that primary amines are the best candidates for CO2 capture from air. Primary amines possess both the highest amine efficiency for CO2 adsorption as well as enhanced water affinity compared to other amine types or the bare silica support. The results suggest that the rational design of amine adsorbents for the extraction of CO2 from ambient air should focus on adsorbents rich in primary amines.