•Heavier lanthanides were preferentially extracted by Cyanex 302 at high metal loading.•The stoichiometry of the complex was LnA3·mH2O across the Ln(III) series.•The coordination structures of the lighter and heavier lanthanides were different.•IR and EXAFS data suggested more O and less S were coordinated to heavier lanthanides.•Coordination difference was due to decreasing covalent binding of 4f electrons toward S.The extraction of trivalent lanthanides (LnIII) 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 NdIII and other LnIII (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 LnIII 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 LnA3·mH2O across the LnIII 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.