Pivotal degradation pathways of the most active non-heme iron epoxidation catalyst to date are investigated in detail. In-depth HR-ESI-MS and NMR spectroscopy decomposition studies and comparative catalytic experiments exclude the generally accepted literature paths of μ2-oxo bridged FeIII–O–FeIII dimer formation and methylene bridge oxidation. Instead, the Fe–NHC bond has been identified as the “weak spot”. It is shown that direct oxidation of one of the carbenes (ImC[double bond, length as m-dash]O) results in its de-coordination and induces protonation of the other NHC moieties, thus completely deactivating the catalyst. Evidence for protonation prior to carbene oxidation has not been found in this study.