Abstract: HDEHP with different kinds of diluent may have different polymeric forms, which in the extraction complexes are normally deduced according to their polymeric constants. In this paper, a method is proposed for the determination of the actual polymeric forms of the HDEHP extracting metal-ions. The distribution coefficients of the ions between the organic and aqueous phases (D) are measured as the function of ion concentrations (Mm+). Based on these data, the extraction capacity of HDEHP (CHdAn) can be calculated with the following formulations. Ch0nAn=mMm+D/1-(D/D,)1/mBy comparision of the extractive capacity (cH0nAn) with the content or concentration of HDEHP(cH0A), the polymeric form is determined. If cH0An is half of the cH0A, the HDEHP is in dimeric form,and if they are equal to each other, the HDEHP extracting metal ions is in monomeric form.By this proposed method, the extractant is not necessarily saturated with metal ions for measuring extractive capacity.lt is shown that the HDEHP with chloroform or without diluent such as HDEHP-levextrel is in monomeric form and that with kerosine is in dimeric form when extracting Eu3+ in nitric acid.