Extensive ab initio calculations have been carried out to study the equilibrium structure, vibrational frequencies, and bonding characteristics of various uranyl (UO22+) and uranyl silicate complexes (UO2SiO4H2 and UO2Si2O7H4). The geometry optimization and calculated vibrational frequencies are discussed and compared to experiment. We have found two types of bridged structures for uranyl silicates consistent with the EXAFS study of U(VI) sorption onto silica gel, which revealed two different bridged structures at different U loadings. (C) 2003 Elsevier Science B.V. All rights reserved.