Foam fractionation is used to selectively extract one alkali-metal ion from a mixed electrolyte through a combination of preferential complexation by an added macrocycle and selective binding of the macrocyclic complex to a surfactant film. Solution and interfacial equilibrium considerations are used to determine optimal conditions for selective extraction, and the predictions tested using two typical macrocycles, Cryptand 222 and 18-crown-6. Proof of principle of tunable extraction is illustrated by the change in selectivity of alkali-metal ions in the presence of these two ligands.