Mechanisms for the rhodium-catalyzed oxidation of olefins have been a matter of debate for along time. So far, mainly closed-shell intermediates such as metallo-oxetanes, metallo-peroxides and metallo-dioxolanes have been proposed as intermediates in these catalytic cycles. Nevertheless some reports were published in which was indicated that open-shell species. such as rhodium superoxides may well be involved in such reaction mechanisms. In this paper an overview is given of catalytic and stoichometric oxidation of substrates at open-shell rhodium and iridium complexes. Rhodium and iridium superoxide complexes have been observed at low temperatures or at room temperature, when enough steric bulk is provided. The chemistry and properties of these species are discussed. Furthermore, the involvement of open-shell species and especially rhodium and iridium superoxides. in oxygenation of coordinated olefins, non-innocent ligands and external substrates starting from both closed-shell as well as open-shell starting materials. is reviewed. (c) 2006 Elsevier B.V. All rights reserved.