The selective epoxidation of olefins with hydrogen peroxide (H2O2) over transition metal substituted zeo-lites is less environmentally impactful than epoxidation schemes that use chlorinated or organic oxidants. The structure and reactivity of reactive intermediates derived from H2O2 and the mechanism for olefin epoxidation on such materials are debated. Here, cyclohexene oxide formation and H2O2 decomposition rates (measured as functions of reactant and product concentrations) and in situ infrared (IR) and UV-vis spectroscopy are used to probe the intervening elementary steps for cyclohexene (C6H10) epoxidation and the identity of the reactive intermediates on a Nb-beta catalyst. IR and UV-vis spectra acquired in situ show that the reactive intermediates are predominantly superoxide species (Nb-Iv-(O-2)(-), observed also by X-ray photoelectron spectroscopy), which form by the irreversible activation of H2O2 over Nb centers. Similar M-(O-2)* (M = Ti or Ta) intermediates were previously assumed to form via reversible processes; however, in situ IR and UV-vis measurements directly show that Nb-Iv-(O-2)(-) forms irreversibly in both H2O and acetonitrile. Activation enthalpies (Delta H double dagger) for C6H10 epoxidation are 27 kJ mol(-1) higher than for H2O2 decomposition, while activation entropies (Delta S double dagger) for epoxidation are 56 J mol(-1) K-1 lower than for H2O2. These comparisons show that the selectivities for epoxidation, via primary reaction pathways, increase with increasing reaction temperatures. Collectively, these results provide a self-consistent mechanism for C6H10 epoxidation that is also in agreement with previously published data. These findings will aid the rational design and study of alternative metal oxide catalysts for olefin oxidation reactions. (C) 2017 Published by Elsevier Inc.