Abstract: The mechanism of the extraction of Am (Ⅲ) and Eu (Ⅲ) by di-(2-ethylhexyl)-phosphoric acid (HDEHP) is studied. The polymer of HDEHP in kerosene and the effect of complexing agent are noted. From plots of logarithm curves of the distribution ratio vs. the equilibrated concentration of a certain extractant molecule and against pH in HNO3 solution, the molecular formula of extraction compounds are found to be AmA3 (HA)3 and EuA3 (HA)3, A-being DEHP-. In DTPA-lactate system, from plots of Jog D+log Y(Yis complexing degree) vs. H+, lactate-, DTPA and HA(0), it is shown that neither DTPA nor L- is contained in the extraction compounds. When pH is less than two, the extraction compound of Am (Ⅲ) is also AmA3 (HA)3.