The peroxides from methylrhenium trioxide (MTO) and hydrogen peroxide, CH3ReO2(eta(2)-O-2), A, and CH3Re(O)(eta(2)-O-2)(2)(H2O), B, have been fully characterized in both organic and aqueous media by spectroscopic means (NMR and UV-vis). In aqueous solution, the equilibrium constants for their formation are K-1 = 16.1 +/- 0.2 L mol(-1) and K-2 = 132 +/- 2 L mol(-1) at pH 0, mu = 2.0 M, and 25 degrees C. In the presence of hydrogen peroxide the catalyst decomposes to methanol and perrhenate ions with a rate that is dependent on [H2O2] and [H3O+]. The complex peroxide and pH dependences could be explained by one of two possible pathways : attack of either hydroxide on A or HO2- on MTO. The respective second-order rate constants for these reactions which were deduced from comprehensive kinetic treatments are k(A) = (6.2 +/- 0.3) x 10(9) and k(MTo) = (4.1 +/- 0.2) x 10(8) L mol(-1) s(-1) at mu = 0.01 M and 25 degrees C. The plot of log ky : versus pH for the decomposition reaction is linear with a unit slope in the pH range 1.77-6.50. The diperoxide B decomposes much more slowly to yield O-2 and CH3ReO3. This is a minor pathway, however, amounting to <1% of the methanol and perrhenate ions produced from the irreversible deactivation at any given pH. Within the limited precision for this rate constant, it appears to vary linearly with [OH-] with k = 3 x 10(-4) s(-1) at pH 3.21, mu = 0.10 M, and 25 degrees C. Without peroxide, CH3ReO3 is stable below pH 7, but decomposes in alkaline aqueous solution to yield CH4 and ReO4-. As a consequence, the decomposition rate rises sharply with [H2O2], peaking at the concentration at which [A] is a maximum, and then falling to a much smaller value. Variable-temperature H-1 NMR experiments revealed the presence of a labile coordinated water in B, but supported the anhydride form for A.