Hydrocarbon oxidations by bio-inspired nonheme iron catalysts and H 2O2 have been proposed to involve an FeIII-OOH intermediate that decays via a water-assisted mechanism to form an Fe V(O)(OH) oxidant. Herein we report kinetic evidence for this pathway in the oxidation of 1-octene catalyzed by [FeII(TPA)(NCCH 3)]2+ (1, TPA = tris(2-pyridylmethyl)amine). The (TPA)FeIII(OOH) intermediate 2 can be observed at -40 C and is found to undergo first-order decay, which is accelerated by water. Interestingly, the decay rate of 2 is comparable to that of product formation, indicating that the decay of 2 results in olefin oxidation. Furthermore, the Eyring activation parameters for the decay of 2 and product formation are identical, and both processes are associated with an H2O/D2O KIE of 2.5. Taken together with previous 18O-labeling data, these results point to a water-assisted heterolytic O-O bond cleavage of 2 as the rate-limiting step in olefin oxidation.