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Dihydrogen trioxide (HOOOH) is formed nearly quantitatively in the low-temperature (−70 °C) methyltrioxorhenium(VII) (MTO)-catalyzed transformation of silyl hydrotrioxides (R3SiOOOH), and some acetal hydrotrioxides, in various solvents, as confirmed by 1H, and 17O NMR spectroscopy. The calculated energetics (B3LYP) for the catalytic cycle, using H3SiOOOH as a model system, is consistent with the experimentally observed activation energy (9.5 ± 2.0 kcal/mol) and a small kinetic solvent isotope effect (kH2O/kD2O = 1.1 ± 0.1), indicating an initial concerted reaction between the silyl hydrotrioxide and MTO in the rate-determining step. With the addition of water in the next step, the intermediate undergoes a σ-bond metathesis reaction to break the Re-OOOH bond and form HOOOH, together with the second dihydroxy intermediate. The final step in the catalytic cycle involves a second, catalytic water that lowers the barrier to form H3SiOH and MTO.