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At variance with previously known coordination compounds, the polyoxometalate (POM)‐embedded ZrIVand HfIVperoxides with formula: [M2(O2)2(α‐XW11O39)2]12−(M=ZrIV, X=Si (1), Ge (2); M=HfIV, X=Si (3)) and [M6(O2)6(OH)6(γ‐SiW10O36)3]18−(M=ZrIV(4) or HfIV(5)) are capable of oxygen transfer to suitable acceptors including sulfides and sulfoxides in water. Combined 1H NMR and electrochemical studies allow monitoring of the reaction under both stoichiometric and catalytic conditions. The reactivity of peroxo‐POMs 1–5is compared on the basis of substrate conversion and kinetic. The results show that the reactivity of POMs 1–3outperforms that of the trimeric derivatives 4and 5by two orders of magnitude. Reversible peroxidation of 1–3occurs by H2O2addition to the spent catalysts, restoring oxidation rates and performance of the pristine system. The stability of 1–3under catalytic regime has been confirmed by FT‐IR, UV/Vis, and resonance Raman spectroscopy. The reaction scope has been extended to alcohols, leading to the corresponding carbonyl compounds with yields up to 99 % under microwave (MW) irradiation. DFT calculations revealed that polyanions 1–3have high‐energy peroxo HOMOs, and a remarkable electron density localized on the peroxo sites as indicated by the calculated map of the electrostatic potential (MEP). This evidence suggests that the overall description of the oxygen‐transfer mechanism should include possible protonation equilibria in water, favored for peroxo‐POMs 1–3.