The selective binding of alkali metal and quaternary ammonium ions to anionic surfactant films at the air/solution interface has been investigated using the ion flotation technique. Surfactant anions studied were the dodecyl phosphate monoanion, the hexadecyl phosphate dianion, bis(2-ethylhexyl) phosphate, dodecanoate, and dodecyl sulfate. Alkali metal ions display only weak selectivity at adsorbed films of all surfactants studied. The binding strength increases in the order Na+ < K+ < Rb+ < Cs+, with the exception of films of dodecanoate, for which the ions have equal affinity, and the hexadecyl phosphate dianion, for which the selectivity sequence is Cs+ < Rb+ < K+ < Na+. By contrast, the binding of quaternary ammonium counterions to adsorbed films of anionic surfactants has a striking dependence on the surfactant headgroup. Results are interpreted using a model which includes solvation of the headgroups and competing counterions and is compared with competitive binding of phosphate, sulfate, and acetate ions at cationic surfactant films.