[en] The extractions of the selected rare earths (Sc, Y, La and Gd) from hydrochloric acid solutions have been investigated using bis(2,4,4-trimethylpentyl)-mono thiophosphinic acid (Cyanex 302, HL) in heptane as an extractant. The results demonstrate that the extractions of rare earths occur via the following reaction:Sc(OH)²⁺+2[(HL)₂]_(O) ↔ [Sc(OH)L₂.2(HL)]_(O)+2H⁺Y³⁺+3[(HL )₂]_(O) ↔ [Y(HL₂)₃]_(O)+3H⁺La(OH)₂⁺+3[(HL)₂]_(O) ↔ [La(OH)₂L.5(H L)]_(O)+H⁺Gd(OH)²⁺+3[(HL)₂]_(O) ↔ [Gd(OH)L₂.4(HL)]_(O)+2H⁺The pH_1/2 values and equilibrium constants of the extracted complexes have been deduced by taking into account the aqueous phase complexation of the metal ion with hydroxyl ligands and plausible complexes extracted into the organic phase. According to the pH_1/2 values, it is possible to realize mutual separation among Sc(III), Y(III), La(III) and Gd(III) with Cyanex 302 by controlling aqueous acidity