Solvent extraction of microamounts of calcium and strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of 2,6-(diphenylphosphino)pyridine dioxide (DPPPDO, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, HL+2, CaL2+2, CaL2+3, SrL2+2, SrL2+3 and SrL2+4 are extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene saturated with water have been determined.