[en] In this study we investigated the potential competitive influence of Y"3"+ on the uptake of trivalent lanthanides and actinides on corundum (α-Al_2O_3). Both batch sorption and time-resolved laser fluorescence spectroscopic (TRLFS) data showed a decreased uptake of Eu"3"+ or Cm"3"+, respectively, in the presence of Y"3"+, pointing toward sorption competition between the trivalent ions. A change in the actinide speciation on the corundum surface could be observed in the presence of Y"3"+. In extended X-ray absorption fine structure (EXAFS) investigations, the speciation change could be attributed to a systematic transition from a tetradentate Am"3"+ surface complex to a tridentate one, when increasing the total metal ion concentration on the mineral surface.