The elemental composition, molecule, and crystal structure of precipitates obtained from an acetone solution of Ln(NO3)(3) (Ln = Pr + Er or La + Nd) with a mixture of bis- and mono-(2-ethylhexyl) phosphoric acids and their sodium salts were studied. In this study we found enrichment of the precipitate with the heavier lanthanide. The separation factor for the La-Nd couple is much greater than for the Er-Pr couple, and decreases with increasing rare-earth yield in the precipitate. We discuss the significant role of the stability of LnA(3) against HNO3, especially the formation of La complexes with mono-(2-ethylhexyl) phosphoric acid during enrichment.