The mechanism of CO2 adsorption in the amine-functionalized metal-organic framework mmen-Mg2(dobpdc) (dobpdc4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; mmen = N,N′- dimethylethylenediamine) was characterized by quantum-chemical calculations. The material was calculated to demonstrate 2:2 amine:CO2 stoichiometry with a higher capacity and weaker CO2 binding energy than for the 2:1 stoichiometry observed in most amine-functionalized adsorbents. We explain this behavior in the form of a hydrogen-bonded complex involving two carbamic acid moieties resulting from the adsorption of CO2 onto the secondary amines. © 2013 American Chemical Society.