Extraction of microamounts of europium and americium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of bis(diphenylphosphino)acetylene dioxide (DPPADO, L) was investigated. The equilibrium data were explained assuming that the species HL+, ML3+, ML23+, and ML33+ (M3+ = Eu3+, Am3+) are extracted into the organic phase. The values of extraction and stability constants of the complex species in nitrobenzene saturated with water were determined. It was found that the stability constants of the corresponding complexes EuLn3+ and AmLn3+, where n = 1,2,3 and L is DPPADO, in water-saturated nitrobenzene are comparable or the same, respectively.