An experimental investigation is presented on flotation of Co(II), Sr(II), and Cs(I) cations from very dilute aqueous solutions with proton-ionizable crown ethers: monoethyl sym (butyl)dibenzo-16-crown-5-oxymethylphosphonic acid, sodium 3-[sym-(decyl) dibenzo-16-crown-5-oxy]propanesulfonate, and sym-(decyl) dibenzo-16-crown-5-oxyacetic acid. used separately or together with a nonionic foaming agent (Triton X-100). The effect of major process variables on both the efficiency and the rate of flotation of Sr(II) cations with sym-(decyl)dibenzo-16-crown-5-oxyacetic acid and Triton X-100 were established. Moreover, the competitive Sr2+/Cs+ flotation was studied with sulfonic and carboxylic 16-crown-5 derivatives. The carboxylic collector exhibited strong affinity towards Sr(II) cations and up to 1000-fold molar excess of lithium, ammonium. and potassium chlorides, and nitrates did not stop flotation of Sr(II) cations. Such affinity allowed for complete separation of Sr2+ from Cs+ in a single-stage batch flotation. Selectivity of flotation of Sr(II), Cs(I), and Co(II) cations with the sulfonic and carboxylic collectors did not follow the "crown ether cavity size" selectivity rule. Nevertheless. the separation of Sr and/or both Sr with Cs from extremely dilute aqueous solutions with these collectors was efficient and fast, which might have a practical meaning for the decontamination of aqueous solutions containing radioactive isotopes. Proton-ionizable crown ether derivatives of sufficient surface activity and water solubility could make a new generation of specific collectors for flotation of aqueous ions.