We studied the structure and stoichiometry of aqueous uranyl(VI) hydroxo dimers and trimers by spectroscopic (EXAFS, FTIR, UV-vis) and quantum chemical (DFT) methods. FTIR and UV-vis spectroscopy were used for the speciation of uranyl complexes in aqueous solution. DFT calculations show that (UO2)(2)(OH)(2)(2+) has two bridging hydroxo groups with a U-U distance of 3.875 angstrom. This result is in good agreement with EXAFS, where a U-U distance of 3.88 angstrom was found,. For the hydroxo trimer complex, DFT calculations show that the species (UO2)(3)-(O)(OH)(3)(+) with oxo bridging in the center is energetically favored in comparison to its stoichiometric equivalent (UO2)(3)(OH)(5)(+). This is again in line with the EXAFS result, where a shorter U-U distance of 3.81-3.82 angstrom and evidence for oxo bridging in the center were found. Several stable intermediates which lie several tens of kJ/mol above that of (UO2)(3)(O)(OH)(3)(+) were identified, and their structures, energies, and intramolecular proton-transfer reaction are discussed.