- Conventional wisdom maintains that β isomers of fully oxidized Keggin heteropolytungstates, [Xn+WVI12O40](8-n)- (X = main-group or transition-metal cation), are unstable with respect to α structures such that isomeric rearrangements all occur in the direction β → α. Contrary to this view, equilibria between α and β forms of the Keggin anion [AlIIIW12O40]5- (α- and β-1) have now been observed. Moreover, a trend in kinetic and thermodynamic stabilities of β isomers in the order X = Al(III) > Si(IV) > P(V) has been established, and the difference in energy between α and β isomers (α- and β-1) has been quantified for the first time. Mild acid condensation of WO42-, followed by addition of Al(III), gave [Al(AlOH2)W11O39]6- (2)three β-isomer derivatives, β1 (Cs symmetry), β2 (C1), and β3 (Cs), with the α derivative (Cs) a minor productin nearly quantitative yield by 27Al NMR spectroscopy. Acidification of the reaction mixture to pH 0 and refluxing cleanly converted 2 to H5[AlIIIW12O40] (1)mostly β-1 (yellow, C3v), with α-1 (white, Td) a minor product. Samples of each isomer were isolated by fractional crystallization and characterized by 27Al and 183W NMR, IR, and UV−vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The Al−O bond length in the Td AlO4 group at the center of α-1 (hydrated potassium salt of α-1; final R1 = 3.42%) establishes a trend in X−O bond lengths in the [Xn+O4](8-n)- groups of α-Keggin anions of 1.74(1), 1.64(2), and 1.53(1) Å, respectively, for X = Al(III), Si(IV), and P(V). Equilibria between isomers of 1 were observed by heating separate 0.1 M aqueous solutions of either pure α or β anions under identical conditions. The progress of the reaction was measured, and the relative concentrations of the α and β isomers present at equilibrium were determined by 27Al NMR spectroscopy. First-order rate constants for approach to equilibrium of α- and β-1 at 473 K were k1(α→β) = 7.68 × 10-7 s-1 and k-1(β→α) = 6.97 × 10-6 s-1. The equilibrium ratio of β-1 to α-1 (k1/k-1) was Keq(473 K, 0.1 M 1) = 0.11 ± 0.01. From ΔG = −RT ln Keq, α-1 is more stable than β-1 by 2.1 ± 0.5 kcal mol-1. Controlled hydrolysis of α-1 gave the monolacunary derivative α-Na9[AlW11O39] (α-3; Cs); hydrolysis of β-1 gave β2-3 (C1) as the major product. Thermal equilibration of the lacunary Keggin heteropolytungstates could also be achieved: Independently heated solutions of either α-3 or β2-3 (0.13 M of either isomer in D2O at 333 K; natural pH values of ca. 7) both gave solutions containing α-3 (60%) and a single β-3 isomer of Cs symmetry (40%). Using Keq = 1.5, the two isomers differ in energy by 0.3 kcal mol-1.