The cesium cation, Cs+, is the largest and least electrophilic singly charged metal ion. Isolation of Cs+ salts from aqueous solutions represents a problem in many areas, as there are (a) exploitation of cesium from minerals, (b) reprocessing of nuclear fuels and separation of the strong gamma emitter 137Cs, (c) decontamination of 137Cs containing waste solutions, and (d) preparation of 131/137Cs radioactive probes for a variety of applications. We have found that the fluorinated aryl boronate (FAB) anion [H2NB2(C6F5)6]– allows for 100% selective and quantitative separation of Cs+ from any aqueous solution, affording insoluble Cs[H2NB2(C6F5)6]. The high specificity for Cs+ arises from the specific conformation of the anion which is exclusively present in Cs[H2NB2(C6F5)6]. Thereby, Cs+ is bonded to five anions in a 3D lattice, and the unprecedented coordination number CN = 16 is reached, allowing for enhanced stability of the lattice.