[en] Solvent extraction of ¹⁷⁵Hf and ¹⁸¹Hf(IV) into solutions of HBPHA in CHCl₃, C₂H₂Cl₄, CCl₄ and benzene from mixtures of water with DMSO or DMFA at 2M HClO₄ formal acidity was studied. With diluents containing acidic hydrogen (CHCl₃, C₂H₂Cl₄) the presence of DMSO or DMFA in the extraction system leads to an increased extractibility of the Hf(IV) chelate. Possible reasons for this synergism are discussed. Extractions into benzene and CCl₄, as well as those into CHCl₃ and C₂H₂Cl₄ at high DMSO and DMFA concentrations, are antagonistic. This is caused by the decrease of the partition coefficient of HBPHA, and changes in the solvation of Hf(IV) ion in the polar phase. The latter effect is very strong at high DMSO concentrations, and with macroconcentrations of Hf(IV) finally leads to precipitation of crystalline adducts Hf(ClO₄).xDMSO (x=7-8). (author)