The electronic characteristics and the redox properties of each isomer of PW12O403- depend on the arrangement adopted by the metal-oxide framework. At the DFT/BP86 level, we computed the structures of the five isomers of PW12O403- in oxidized form. The energy scale fits the experimental findings as well as the number of rotated triads of the metal-oxide core since the energy grows as follows: alpha < beta < gamma < delta < epsilon. The reduced clusters behave differently as long as the beta form becomes the most stable isomer after the second reduction. The gamma isomer also gains stability upon reduction, but not enough to be competitive with beta. For the 4-fold reduced PW12 cluster, the energy difference computed between beta and gamma in solution is 11 kcal mol(-1). This large difference proves that the beta --> gamma isomerization is not favored upon simple reduction. The other isomers, delta and , are much more unstable than alpha or beta in any reduction state.