The synergistic extraction of holmium(III) with 2-thenoyltrifluoroacetone (Htta) in the presence of neutral bidentate heterocyclic amines such as 2, 2′-bipyridine (bpy), 4, 4′-dimethyl-2, 2′-bipyridine (dmbpy), and 4, 4′-diphenyl-2, 2′-bipyridine (dpbpy), and a neutral terdentate amine of 2, 2′:6′, 2"-terpyridine (tpy) in benzene has been investigated. The synergism was found to be due to the formation of an adduct complex of Ho(tta)3S (S denotes a heterocyclic amine) in all of the systems studied. The extraction constant and the adduct formation constant (βs, 1) for Ho(tta)3S are given. The values of βs, 1 increased in the order tpy