A family of dimeric, peroxo-containing heteropolytungstates, [M-2(O-2)(2)(XW11O39)(2)](12-) [M = Zr4+, X = Si (1), Ge (2); M = Hf4+, X = Si (3)], have been synthesized by reacting ZrCl4/HfCl4 with the respective monolacunary Keggin precursor [XW11O39](8-) (X=Si, Ge) in an aqueous acidic medium (pH 4.8). The isostructural polyanions 1-3 are composed of two (XW11O39) Keggin units encapsulating a central diperoxo-dimetal fragment {M-2(O-2)(2)}(4+) (M=Zr4+, Hf4+). Cyclic voltammetry and exhaustive electrolysis studies indicate fast reductive release of the peroxo ligands upon reduction of 1-3. Stoichiometric oxo-transfer studies from 1-3 to the substrate L-methionine were performed, and the reactions were monitored by H-1 NMR.