Oxotrimesityliridium(V), (mes)(3)IrO (mes = 2,4,6-trimethylphenyl), and trimesityliridium(III), (mes)(3)Ir, undergo extremely rapid degenerate intermetal oxygen atom transfer at room temperature. At low temperatures, the two complexes conproportionate to form (mes)(3)IrOIr(mes)(3), the 2,6-dimethylphenyl analogue of which has been characterized crystallographically. Variable-temperature NMR measurements of the rate of dissociation of the mu-oxo dimer combined with measurements of the conproportionation equilibrium by low-temperature optical spectroscopy indicate that oxygen atom exchange between iridium(V) and iridium(III) occurs with a rate constant, extrapolated to 20 degrees C, of 5 x 10(7) M-1 s(-1). The oxotris(imido)osmium(VIII) complex (ArN)(3)OsO (Ar = 2,6-diisopropylphenyl) also undergoes degenerate intermetal atom transfer to its deoxy partner, (ArN)(3)Os. However, despite the fact that its metal-oxygen bond strength and reactivity toward triphenylphosphine are nearly identical to those of (mes)(3)IrO, the osmium complex (ArN)(3)OsO transfers its oxygen atom 12 orders of magnitude more slowly to (ArN)(3)Os than (mes)(3)IrO does to (mes)(3)Ir (k(OsOs) = 1.8 x 10(-5) M-1 s(-1) at 20 degrees C). Iridium-osmium cross-exchange takes place at an intermediate rate, in quantitative agreement with a Marcus-type cross relation. The enormous difference between the iridium-iridium and osmium-osmium exchange rates can be rationalized by an analogue of the inner-sphere reorganization energy. Both Ir(III) and Ir(V) are pyramidal and can form pyramidal iridium(IV) with little energetic cost in an orbitally allowed linear approach. Conversely, pyramidalization of the planar tris(imido)osmium(VI) fragment requires placing a pair of electrons in an antibonding orbital. The unique propensity of (mes)(3)IrO to undergo intermetal oxygen atom transfer allows it to serve as an activator of dioxygen in cocatalyzed oxidations, for example, acting with osmium tetroxide to catalyze the aerobic dihydroxylation of monosubstituted olefins and selective oxidation of allyl and benzyl alcohols.