Extraction of microamounts of barium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of Slovafol 909 (L) has been investigated. The equilibrium data have been explained assuming that the species HL+, HL+2, BaL2+ and BaL2+2 are extracted into the organic phase. The values of extraction and stability constants of the complex species in nitrobenzene saturated with water have been determined.