The working capacity, heat of adsorption, and stability of polyamine sorbents were experimentally tuned using a one-step modification with alkyl or epoxy reactants, producing alkyl or hydroxyalkyl functional groups on the polyamine. The stability of the functionalized sorbents can outweigh the lower working capacity in some cases, emphasizing the important role of sorbent stability in economic feasibility. Degradation information coupled with a capacity fade economic model is vital for accurately predicting the cost of direct air capture.