Abstract: The equilibria between Am(Ⅲ), Cm(Ⅲ) and α-hydroxy-α-methylbutyrate (HMBA) were investigated by a cation exchange method at 25℃ and at a constant ionic strength μ=0.5. Sodium perchlorate was used as a supplementary salt in the solution. The radioactivity of americium and curium was determined using a Intertechnique SL-30 liquid scintillation spectrometer with a relative accuracy≤3%.Stoichiometric over-all constants were computed in two ways: by Fronaeous's graphical method and via a weighted least squares technique using a PDF 11/34 computer. The results agree quite well and prove the existence of three mononuclear complexes, viz MA2+, MA+2 and MA3. The over-all stability constants of the complexes formed are: for Am (Ⅲ) β1 = (3.45±0.05) ×102; β2 = (1.59±0.10)×104; β3 = (4.6±0.2)×105.for Cm (Ⅲ) β1 = (3.84±0.06) ×102; β2 = (2.32±0.17)×104; β3 = (8.5±0.3)×105.Since HMBA is part of a series where the amount of alkyl substitution on the alpha carbon varies, there are two different effects that should be taken into account. The first is the basicity of the donor oxygen atoms. The inductive effect of an alkyl group alpha to the carboxylate is to increase the basicity. The second factor is the steric factor. For HMBA, however, the inductive effect is the dominating factor. By comparison of the stability constants of the HMBA, HIBA and lactic complexes (Table 1), it can be expected, that HMBA is superior to HIBA and lactic acid as an eluant for the cation exchange separation of americium and curium. Table 1 The stability constants of some α-hydroxycarboxylate complexes of Am and Cm