Mechanisms have been proposed for alpha-KG dependent non-heme iron enzyme catalyzed oxygen atom insertion into an olefinic moiety in various natural products, but they have not been examined in detail. Using a combination of methods including transient kinetics, Mossbauer spectroscopy, and mass spectrometry, we demonstrate that AsqJ-catalyzed (-)-4'-methoxycyclopenin formation uses a high-spin Fe(IV)-oxo intermediate to carry out epoxidation. Furthermore, product analysis on O-16/O-18 isotope incorporation from the reactions using the native substrate, 4'-methoxydehydrocyclopeptin, and a mechanistic probe, dehydrocyclopeptin, reveals evidence supporting oxo <-> hydroxo tautomerism of the Fe(IV)-oxo species in the non-heme iron enzyme catalysis.