At variance with previously known coordination compounds, the polyoxometalate (POM)-embedded ZrIV and HfIV peroxides 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 1–5 is compared on the basis of substrate conversion and kinetic. The results show that the reactivity of POMs 1–3 outperforms that of the trimeric derivatives 4 and 5 by two orders of magnitude. Reversible peroxidation of 1–3 occurs by H2O2 addition to the spent catalysts, restoring oxidation rates and performance of the pristine system. The stability of 1–3 under 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 1–3 have 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.