The extraction of trivalent lanthanides (Ln(III)) by purified Cyanex 302 (bis(2,4,4-trimethylpentyl)monothiophosphinic acid, denoted as HA) at high and constant metal loading was studied by the co-extraction of Nd-III and other Ln(III) (where Ln = La to Lu, except Pm), and the results were compared with that of unpurified compound. Heavier lanthanides were extracted preferentially, and the average separation factor of adjacent lanthanides by HA was slightly lower than that by unpurified Cyanex 302. The complexation of Ln(III) with HA in the organic phase was further investigated by absorption/fluorescence spectroscopic titration, and IR and extended X-ray absorption fine structure spectroscopy. Under the studied experimental conditions, the stoichiometry of the major extracted complex was found to be LnA(3)center dot mH(2)O across the Ln(III) series, but the coordination structures of the lighter and heavier lanthanides were different. The data suggested that a greater number of sulfur atoms coordinate to lighter lanthanides than to heavier lanthanides. (C) 2014 Elsevier B.V. All rights reserved.