Abstract The effect of the chelating ligands on the stability of mixed chelates of lutetium with β-diketones and the stability of their adducts with TOPO in benzene was studied by extracting lutetium with mixtures of two β-diketones, benzoylacetone (BzA) and dibenzoylmethane (DBM), and benzoylacetone and benzoyltrifluoroacetone (BFA), in the absence or in the presence of TOPO. The results are summarized as follows. (1) The extraction constants of Lu-BzA-DBM mixed chelates are almost the same as statistically calculated, while those of Lu-BzA-BFA mixed chelates are somewhat lower than the theoretical values. (2) The stability of the TOPO adducts of the mixed chelates increases in the order, M(BzA)2(DBM)≈M(BzA)(DBM)2<M(BzA)2(BFA)<M(BzA)(BFA)2, which is almost the same as that found among the extraction constants of the mixed chelates. The fluoromethyl group stabilizes the adduct more than phenyl group. (3) All the mixed chelates of lutetium β-diketonates bind one molecule of TOPO per metal chelate.