The distribution of microamounts of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous HNO3 solutions and solutions of bis(dioctylphosphinylmethyl)phosphinic acid in organic diluents was studied. The stoichiometry of the extractable complexes was determined, and the diluent effect on the efficiency of extraction of rare-earth elements from nitric acid solutions was examined. Addition of a neutral solvating component, tetraphenylmethylenediphosphine dioxide, to the organic phase enhances the extraction of rare-earth elements, exerting a synergistic effect.