Cu-di-(2-ethylhexyl)phosphate was used as the cation exchange extractant from molten nitrate salt. IR absorption spectra of di-(2-ethylhexyl)phosphoric acid and Cu-di-(2-ethylhexyl)phosphate were compared and it was proved that the acidic form of the extractant is not Cu-di-(2-ethylhexyl)-phosphate. Using LiNO3-NH4NO3 eutectic melt, it was shown that the back-extraction of Cu2+ is a cation exchange reaction. Np(V) and Pa(V) were extracted by Cu-di-(2-ethylhexyl)phosphate from LiNO3-NaNO3-KNO3 eutectic melt. The distribution ratio of Np(V) was greater than that of Pa(V) on the contrary of their distribution ratios in the aqueous extraction system. A possible cation exchange extraction reaction was proposed for the extraction of Np(V).