[en] The extraction of Am³⁺, Eu³⁺, Tm³⁺ and Y³⁺ from an aqueous chloride phase into a solution of bis 2-ethylhexyl phosphoric acid, (2-C₂H₅.C₆H₁₂O)₂PO(0H), HDEHP, in the monomerizing diluent 2-ethylhexanoic acid has been studied in terms of the concentration of hydrogen ion in the equilibrated aqueous phase and of HDEHP in the opposing equilibrated organic phase. The extraction stoichiometries with the corresponding expression for K in terms of F(the formality of HDEHP in the organic phase), [H⁺] (the concentration of hydrogen ion in the aqueous phase), and Ksub(S), for a 1.00 F(NaCl + HCl) aqueous phase are: Am³⁺, Eu³⁺: Msub(A)³⁺ + 3HY₀ =MY₃₀ + 3Hsub(A)⁺ (K Ksub(S)F³/[H⁺]³); and Tm³⁺, Y³⁺: Msub(A)³⁺ + 5HY₀ = M(H₂Y₅)₀ + 3Hsub(A)⁺ (K = Ksub(S)F⁵/[H⁺]^-3), where HY represents the monomer of HDEHP and the formulations of extracted species are non-committal with respect to structure. With HZ representing the monomer of 2-ethylhexanoic acid, suggested formulations of the extracted species in terms of homogeneous and heterogeneous dimers are: for Am³⁺ and Eu³⁺, M(HYZ)₃; and for Tm³⁺, Y³⁺, M(HY₂)₂(HYZ). The Ksub(S) values are: Am³⁺(5 x 10^-3), Eu³⁺(1 x 10^-2), Tm³⁺(7) and Y³⁺(2). (author)