Extraction of microamounts of barium by a nitrobenzene solution of ammonium dicarbollylcobaltate (NH4+B-) in the presence of 15-crown-5(15C5,L) has been investigated. The equilibrium data have been explained assuming that the complexes NH4L+, NH4L2+ and BaL22+ are extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene saturated with water have been determined.