The unimolecular reactions of ''bare'' first-row transition-metal cations Cr+-Co+ complexed to small alkyl hydroperoxides and hydrogen peroxide have been examined by means of tandem mass spectrometry. In the metal-mediated reactions of the methyl, ethyl, and isopropyl hydroperoxides disproportionations of the peroxides dominate which involve insertion of the metal ion into the weak O-O bond followed by beta-hydrogen shifts, These processes lead to competitive losses of water, aldehydes, or ketones as thermochemically favored neutral products. In addition, for CH3OOH/M(+) unimolecular dehydrogenation to yield M(OH2)(CO)(+) takes place, The metal complexes of tert-butyl hydroperoxide give rise to methanol as a side product via an analogous beta-methyl migration and subsequent reductive elimination of methanol, However, for this complex the major reaction channel corresponds to water loss, which is rationalized by a multistep rearrangement, Interestingly, the observed reactivity patterns remain essentially unchanged irrespective of the nature of the metal, This may be attributed to the high thermodynamic stability of the products and the absence of other competing reaction channels, In contrast to the alkyl hydroperoxides, the hydrogen peroxide/M(+) complexes permit oxygen-atom transfer to the metal center to yield inter alia MO(OH)(+), M(OH)(+), and MO(+) cations. As shown previously, the latter can further be used as oxidants in the oxygenation of hydrocarbons.