Extraction coefficients and separation factors of all lanthanides in the diluted aqueous HDBP—HNO3 system were determined in the temperature range 10–50 °C. Essential differences were noticed between the results obtained
for light (La—Gd) and heavy (Gd—Lu) lanthanides. It was found that the selectivity of the investigated system, in the case
of heavy lanthanides, is as high as that observed for the best organophosphorous extractants: HEHϕP and HDEHP. The sequence
of the separation factors shows the pattern of regularities (the double-double effect) within the whole series of lanthanides
and within the whole investigated temperature range.