The synergistic solvent extraction of Pr, Gd, and Yb with mixture of thenoyltrifluoroacetone (HTTA) or 1-phenyl-3-methyl-benzoyl-pyrazol-5-one (HP) and tridodecylammonium chloride (TDAHCl) in C6H6 has been studied. The composition of the extracted complexes has been determined asLn(TTA)3TDAHCl andTDAH+[LnP4]−. The values of the equilibrium constants have been calculated. Some peculiarities of the synergistic extraction process are discussed.