Metastable Mn-peroxo species are proposed to form as key intermediates in biological oxidation reactions involving O-2 and C-H bond activation. The majority of these have yet to be spectroscopically characterized, and their inherent instability, in most cases, precludes structural characterization. Cysteinate-ligated metal-peroxos have been shown to form as reactive intermediates in both heme and nonheme iron enzymes. Herein we report the only examples of isolable Mn(III)-alkylperoxo species, and the first two examples of structurally characterized synthetic thiolate-ligated metal-peroxos. Spectroscopic data, including electronic absorption and IR spectra, and ESI mass spectra for O-16 vs O-18-labeled metastable Mn(III)-OOR (R = Bu-t, Cm) are discussed, as well as preliminary reactivity.