Dioxygen is used as the oxygenation agent in the rhodium-mediated conversion of nitriles into amides. The characterization of intermediate species and model compounds as well as isotope-labeling studies provided an insight into the reaction mechanism. The conversions of rhodium hydroperoxido or methylperoxido complexes with nitriles into metallacyclic rhodium-κ(2)-(N,O)-peroxyimidate compounds represent essential key steps. The former are accessible from a rhodium(III) peroxido complex and the latter represent rhodium derivatives of Payne's reagent (peroxyimidic acids).