Abstract The synergistic effects of n-hexyl alcohol, TBP and TOPO on the extraction of rare earth ions with benzoyltrifluoroacetone in benzene have been studied. The stability constants of the adducts were determined by means of the curve-fitting method. The equation based on the shift of pH1⁄2 (−ΔpH1⁄2) was also employed to analyse the data. The results were discussed in relation to the ionic radius of rare earth elements and summarized as follows: (1) The stability of rare earth benzoyltrifluoroacetonate decreases but that of the adducts increases, as the ionic radius increases. (2) The maximum number of base molecules bonding to one molecule of the metal chelate is 2, except that in the case of lutetium adduct with TOPO, where the formation of secondary adducts is not observed. (3) Although the overall stability constant of the adduct increases in the following order; lutetium < terbium < europium < lanthanum, as mentioned above, the first stability constant does not show such a distinct sequence. Therefore the contribution of the second stability constant seems to be an important factor. (4) Organic bases studied form stable adducts with rare earth benzoyltrifluoroacetonates in the following order; n-hexyl alcohol<TBP<TOPO. This order corresponds to the donating power of lone pair electrons of the oxygen atoms in these compounds.