U L(m)-edge EXAFS spectra were measured for 1:1 mixtures of the uranyl ion with tartaric, malic, and citric acids. The room-temperature EXAFS spectra of these mixtures demonstrate the characteristic uranium-oxygen interactions of the uranyl ion UO22+ and also show a U-U interaction at 3.92 Angstrom, indicating the presence of a uranyl dimer. On the basis of models proposed by Martell et al. from earlier potentiometric titration experiments, it is concluded that the dimer is the ligand-bridged species (UO2)(2)(L)(2), in which bridging occurs through the alpha-hydroxyl groups of the polycarboxylate ligands.