The synergistic solvent extraction of selected trivalent lanthanoids (La, Nd, Eu, Ho and Lu) with mixtures of a chelating extractant 4-benzoyl-3-phenyl-5-isoxazolone (HBPI) and tridodecylammonium perchlorate (TDDAHClO(4)) or methyldioctylammonium perchlortae (MDOAHClO(4)) in C6H6 has been studied. The composition of the extracted complexes has been determined as TDDAH[Ln(BPI)(4)] or MDOAH[Ln(BPI)(4)]. The values of equilibrium constants as well as those of synergistic coefficients and separation factors were calculated. The extraction mechanism has been discussed. (C) 2010 Elsevier B.V. All rights reserved.