SummaryThe synergistic solvent extraction of trivalent lanthanides such as Ce(III), Tb(III) and Lu(III) has been studied with a mixture of hexafluoroacetylacetone (HHFA) acting as an acidic chelating agent and triphenylphosphine oxide (TPPO) as a neutral ligand in benzene from aqueous solution of pH 2.5 having ionic strength 0.01 mol dm−3(K+/H+, Cl−). The composition of the synergistic adduct has been determined by slope analysis method and found to be M(HFA)3·2TPPO [M=Ce(III), Tb(III) Lu(III)]. The effect of various anions and cations on the extraction of these metal ions has also been studied and only oxalate among the anion and Cr(III) among the cations has some deleterious effect. The formation constant and separation factor from various metal ions have also been determined and discussed.