To determine the relative stability of alpha and beta rotational isomers of the Wells-Dawson structure, the energies of some fully oxidized, single- and 2-fold-reduced systems were calculated by means of DFT calculations. The thermodynamics of the alpha/beta equilibrium for P2M18 Wells-Dawson anions is slightly shifted toward the a structure, but the difference in stability is smaller than in the Keggin anions. Tungstates (2:18) and vanadotungstates (2:3:15) show minimal redox differences between isomers, as the electronic structure of the frontier orbitals appears to be nearly the same. In addition, an alternative arrangement is proposed that have long and short Mo-O bonds in beta-P2Mo18 with an idealized C-3 symmetry. This arrangement was computed to be about 8.2 kcal mol(-1) more stable than the nonalternate framework of C-3nu symmetry. The P2Mo18 is the Wells-Dawson anion for which the alpha/beta equilibrium most resembles that of the Keggin anions.