Compared to the system in aqueous solution, the equilibration reactions in acetonitrile between MTO and the methylrhenium peroxides CH3ReO2(eta(2)-O-2) (A) and CH3ReO(eta(2)-O-2)(2)(H2O) (B) are slower but more favored thermodynamically. In CH3CN, small concentrations of water facilitate the formation of A (especially) and B. These species decompose to methyl hydroperoxide and perrhenic acid in CD3CN, rather than to methanol and perrhenic acid as in aqueous solution. The proposed mechanism involves the intramolecular migration of the methyl group to a peroxo oxygen, followed by hydrolysis, and it is facilitated by photolysis. The potential use of B as photocatalyst does not seem promising, however.