The synergistic solvent extraction of lanthanide ions with Mixtures of 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (HP) and the quaternary ammonium salt Aliquat 336 in chloride (QCl) or perchlorate (QClO(4)) forms in C6H6 was studied. The composition of the extracted species was determined as Q[LnP(4)] (Q(+) is a quaternary ammonium salt cation). The values of the equilibrium constant were calculated. The extraction mechanism as well as the effect of the quaternary ammonium salt anion (Cl- or ClO4-) on the extraction process are discussed. The separation factors of the lanthanide ions were determined on the basis of the experimental data. Some trends in the lanthanide separation were established.