Abstract The steric effect and the inductive effect of the functional groups of β-diketones on the adduct formation between lutetium chelates and TOPO in benzene was studied by using various β-diketones with aliphatic groups (acetylacetone, dipropionylmethane, pivaroylacetone, diisobutyryl-methane and dipivaroylmethane), aromatic groups (benzoylacetone, naphthoylacetone and dibenzoylmethane) and fluoromethyl groups (trifluoroacetylacetone, furoyltrifluoroacetone, thenovltrifluoroacetone, benzoyltrifluoroacetone and hexafluoroacetylacetone). The effect of the temperature on the adduct formation was then estimated. These results may be summarized as follows: (1) The stability of the adduct increases, and the larger synergism appears, as aliphatic groups, aromatic groups, and fluoromethyl groups are successively substituted for the functional groups of β-diketones. This effect may be due to the electron-withdrawing effect of the fluoromethyl group and the conjugation effect (with a metal chelate ring) of the phenyl group. (2) The adducts contain one molecule of TOPO per metal chelate except for the HFA adduct, which contains two molecules of TOPO. The steric hindrance of the terminal groups is hardly recognizable. (3) The extraction constant of the chelates increases, while the stability of the adducts decreases, with a rise in the temperature. The apparent enthalpy change (−ΔH kcal/mol) upon the adduct formation increases in the order: DPM (5.5)<BzA(5.9)<BFA(8.6) chelates.