The synergistic solvent extraction of Pr, Gd and Yb with mixtures of thenoyltrifluoroacetone (HTTA) and primary ammonium salt (tridecylammonium chloride or perchlorate, TDAH(CI, ClO4)) in C6H6 has been studied. The composition of the extracted species have been determined as Ln(TTA)(3)TDAHA(A(-)= Cl- or ClO4-). The values of the equilibrium constant K-T,K-S have been calculated. The influence of the ammonium salt anion on the extraction process has been discussed. The separation factors of the pairs Gd/Pr and Yb/Gd have been determined.