DFT and post Hartree-Fock calculations were carried out to characterize the electronic structure of the 10-electron-reduced [PMo8V4O40(VO)4](5)(-) polyoxometalate. This molecule may be viewed as a mixed-metal PMo8V4O40 Keggin structure capped with four VO units, in which the eight vanadiums form a ring. In mixed V/Mo clusters it is accepted that the first reductions occur at the V(5+) ions. The BP86 calculations on this modified Keggin anion reveal that the ground state is a septet with the six unpaired electrons delocalized over the eight V centers. The B3LYP calculations and especially the CASSCF technique modify the tendency of the BP86 method, thus reproducing the expected 8/2 distribution. The unpaired electrons residing in the eight vanadiums are antiferromagnetically coupled.