Epoxidation of olefins by H2O2 as catalyzed by methylrhenium trioxide, CH3ReO3, was investigated in semi-aqueous and organic solvents. Methylrhenium oxide activates H2O2 through formation of a mono-peroxo-Re(VII), A, and a bisperoxo-Re(VII), B, species. Both species are active toward olefin epoxidation with very similar reactivities and are about 105 times more reactive than free H2O2. Epoxides, the initial products from these reactions, undergo ring-opening to the 1,2-diols. A concerted mechanism that involves external nucleophilic attack of the olefin on the peroxy oxygen has been proposed;The kinetics and the mechanisms of oxidations of 1,2-diols, phenols and H2O2 by the pentaaquaoxochromium(IV) ion, CrO2+ were studied in aqueous acidic solutions. Oxidation of vicinal diols with CrO2+ involves a hydride abstraction from the [alpha]-position (primary and secondary 1,2-diols) or from the [beta]-position (pinacol). Reactions of phenols with CrO2+ produce the corresponding phenoxyl radicals in a rate-determining-step. The phenoxyl radicals are further oxidized by the superoxochromiun(III), CrOO2+, to produce quinones as final products. Oxidation of H2O2 with CrO2+ produces CrOO2+ in the presence and in the absence of O2.