Extraction of calcium by a nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of 15-crown-5 (15C5, L) has been investigated. The equilibrium data and the typical maxima concerning the dependence of the Ca distribution ratios on the analytical concentration of 15C5 in the system under study can be explained assuming that the HL+, HL2+ and CaL2+, CaL22+ particles are extracted into the nitrobenzene phase. The values of the corresponding extraction and stability constants of the extracted species in nitrobenzene saturated with water have been determined and the mutual Sr/Ca separation has been discussed