The selectivity of adsorption between alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) based on the ionic functional groups of the surfactants was studied using two types of surfactants, dodecanoic acid (DA) and sodium dodecyl sulfate (SDS), in the foam separation system. The results showed that Li⁺ was preferably removed by foam separation using DA. The removal rates of other alkali metal ions were relatively low, and there were no significant differences among other alkali metal ions (Na⁺, K⁺, Rb⁺, and Cs⁺). However, Cs⁺ exhibited the highest removal rate among the mixed alkali metals by foam separation using SDS. From these results, the selectivity of the alkali metal in foam separation was dependent on the type of surfactant.