Extensive geometry optimizations have been performed at the BP86 level of density functional theory, in order to identify the most stable isomer of pentacoordinated [VO(OH)UH2O](+) and [VOU(H2O)(2)](2+) as well as of hexacoordinated [VO(OH)U(H2O)(2)](+) and [VOU(H2O)(3)](2+) complexes (U = hydroxyurea anion). Most of these are conformationally very flexible, with up to 12 isomers within an energy range of 5 kcal/mol. The most stable hexacoordinate forms are characterized by the oxo ligand in trans position to the carbonyl O atom of U. Bulk solvent effects on the relative stabilities, estimated from a polarizable continuum model, are indicated to be small and do not affect the energetic sequence of the isomers significantly. Details of the coordination sphere of the most stable isomers in aqueous solution (coordination number, protonation state) have been studied with Car-Parrinello molecular dynamics simulations. The preferred mechanisms of interconversion between selected [VO(OH)U(H2O)(2)](+) isomers, according to the DFT computations, involve proton transfers between H2O and OH or between O and OH ligands in the coordination sphere of the metal, assisted by a water molecule from the second hydration sphere.