[en] The extraction of cations in water - nitrobenzene - bis-1,2-dicarbollylcobaltate system usually proceeds as cation exchange between aqueous and organic phases. Only heavy alkali metal cations can be extracted without the presence of ligand. The distribution ratios of other metal ions in aqueous solution of mineral acid - H⁺ form of bis-1,2-dicarbollylcobaltate in nitrobenzene system are too low, so these ions must be extracted as more hydrophobic complexes. The most used ligands are cyclic and linear polyoxyethylene compounds. We have established that the extraction process is a mutual exchange of metal cations and polyoxyethylene compounds metal complex cations between the aqueous and nitrobenzene phases. The equilibrium constants take place during the metal extraction in the system were studied. The results obtained during this study in the field of extraction of water - nitrobenzene - crown extraction system are presented. As the selectivity of alkali earth metal cations increase in the order Ca²⁺< Sr²⁺< Ba²⁺ and the stability constants of the same ions with complexones increase in order Ba²⁺< Sr²⁺ < Ca²⁺ the addition masking agents increases the selectivity of separation by two orders of magnitude