Nowadays, separation of trivalent lanthanides and actinides is considered to be a key issue in the advanced spent nuclear fuel reprocessing. In this work, a series of possible extraction complexes of Am3+ and Eu3+ with the traditional organophosphorus ligand HDEHP (di-(2-ethylhexyl) phosphoric acid) in nitric acid solution have been systematically explored by using scalar-relativistic density functional theory (DFT). Based on our calculations, HDEHP can coordinate with M3+ (M=Eu, Am) cations in the form of hydrogen-bonded dimers HL2- (L=DEHP–), and these dimers prefer to act as bidentate ligands in all the studied complexes. According to thermodynamic analysis, the HL2- dimer shows comparable affinity for Eu(III) and Am(III). The 1:3 type complexes M(HL2)3 seem to be more stable in the organic phase than the 1:1, 1:2 type nitrate complexes due to the higher lipophilicity of the former (see the article by Juan Luo, Congzhi Wang, Jianhui Lan, Qunyan Wu, Yuliang Zhao, Zhifang Chai, Changming Nie & Weiqun Shi on pages 324–331).