The synergistic solvent extraction of Pr, Gd, and Yb with mixtures of 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (HP) and tridodecylammonium (TDAHA), trioctylammonium (TOAHA), and dioctylammonium (DOAHA) salts (A −=Cl−, NO 3 − , and ClO 4 − ) in C6H6 has been studied. The composition of the extracted species has been determined asAmH+[LnP 4]− (AmH+: ammonium salt cation). The values of the equilibrium constant have been calculated. The influence of the ammonium salt ions on the extraction process is discussed.