The structural and electronic properties of monoperoxo and diperoxo uranyl complexes with aquo, fluoride, hydroxo, carbonate, and nitrate ligands have been studied using scalar relativistic density functional theory (DFT). Only the complexes in which the peroxo ligands are coordinated to the uranyl moiety in a bidentate mode were considered. The calculated binding energies confirm that the affinity of the peroxo ligand for the uranyl group far exceeds that of the F-, OH-, CO32-, NO3-, and H2O ligands. The formation of the monoperoxo complexes from UO2( H2O)(5)(2+) and HO2- were found to be exothermic in solution. In contrast, the formation of the monouranyl-diperoxo, UO2(O-2)(2)X-2(4-) or UO2(O-2)(2)X4-/3- (where X is any of F-, OH-, CO32-, or NO3-), complexes were all found to be endothermic in aqueous solution. This suggests that the monoperoxo species are the terminal monouranyl peroxo complexes in solution, in agreement with recent experimental work. Overall, we find that the properties of the uranyl-peroxo complexes conform to well-known trends: the coordination of the peroxo ligand weakens the U-O-yl bonds, stabilizes the sigma(d) orbitals and causes a mixing between the pi- and peroxo sigma- and pi-orbitals. The weakening of the U-O-yl bonds upon peroxide coordination results in uranyl stretching vibrational frequencies that are much lower than those obtained after the coordination of carbonato or hydroxo ligands.