Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO center dot, ROOH, and RO center dot). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples CraqOO2+/CraqOOH2+ and L-1(H2O)RhOO2+/L-1(H2O)RhOOH2+ was estimated to be 10(1 +/- 1) M-1 s(-1). The use of this value in the simplified Marcus equation for the CraqO2+/CraqOOH2+ cross reaction provided an upper limit k(CrO,CrOH) <= 10((-2 +/- 1)) M-1 s(-1) for CraqO2+/CraqOH2+ self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O-2 with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.