Abstract <p>Specific features of the preparative extraction-chromatographic separation of small amounts of lutetium and prevalent amounts of ytterbium using a sorbent based on Prefilter resin, impregnated with 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, are considered. In the presence of prevalent ytterbium amounts, the distribution ratios of both ytterbium and lutetium decrease owing to the formation of complexes Ln(HA₂)₃ or LnA(HA₂)₂, which decreases the concentration of the free extractant in the sorbent and is the major factor responsible for the significant deviation of the elution curve shape from the classical Gaussian shape. Analysis of the Lu and Yb distribution along the column and the Fourier IR spectra of the sorbent samples show that the formation of the third phase in the sorbent at the acidity lower than 0.1 M is improbable, and this process does not affect the behavior of Yb and Lu.</p>