Extraction of microamounts of cesium by the nitrobenzene solution of H+ bis-1,2-dicarbollylcobaltate (H+B−) in the presence of 15-crown-5 (15C5, L) has been investigated. The equilibrium data have been explained assuming the complexes HL+, HL2+, CsL+ and CsL2+ to be extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene saturated with water have been determined.