The lack of information on the structure of peroxovanadium complexes in aqueous solution, together with the difficulty to obtain clearcut experimental evidence, prompted us to approach the problem by carrying out ab initio calculations (HF/3-21G(*) and BSLYP/LANL2DZ). To the naked oxoperoxocation [VO(O-2)](+), whose preferred structure has been determined to be the "cisoid" one, water molecules have been subsequently added and the resulting geometries optimized. The maximum stabilization due to complexation of the aqua ligands is obtained when three molecules of water are coordinated to vanadium. Further coordination to the metal is characterized by a lower hydration energy, similar to that pertaining to the formation of the second solvation shell. Accordingly, the preferred coordination number is six [VO(O-2)(H2O)(3)(+)] with a distorted octahedral geometry and a readily exchangeable fourth water molecule. These results should be compared with those obtained by diffractometric analysis for several monoperoxovanadium complexes in the solid state which exhibit a pentagonal bipyramid geometry and hence a coordination number of seven.