The reaction of [Fe(CF3SO3)2(PyNMe3)] with excess peracetic acid at −40 °C leads to the accumulation of a metastable compound that exists as a pair of electromeric species, [FeIII(OOAc)(PyNMe3)]2+ and [FeV(O)(OAc)(PyNMe3)]2+, in fast equilibrium. Stopped-flow UV/Vis analysis confirmed that oxygen atom transfer (OAT) from these electromeric species to olefinic substrates is exceedingly fast, forming epoxides with stereoretention. The impact of the electronic and steric properties of the substrate on the reaction rate could be elucidated, and the relative reactivities determined for the catalytic oxidations could be reproduced by kinetic studies. The observed fast reaction rates and high selectivities demonstrate that this metastable compound is a truly competent OAT intermediate of relevance for nonheme iron catalyzed epoxidations ; Financial support for this work was provided by the EuropeanCommission (2011-CIG-303522 to A.C. and ERC-2009-StG-239910 to M.C.), the Spanish Ministry of Science (CTQ2012-37420-C02-01/BQU to M.C., CTQ2012-37821-C02-02 to M.G.B., CTQ2013-43012-P to A.C., CTQ2013-48917-C3-1-P to R.B. and E.G.-E.), the Unidad de Excelencia (MDM 2015-0038 and CSD2010-00065 to M.C., M.G.B., R.B., and E.G.-E), the Generalitat Valenciana (PROMETEOII 2015/002t to R.B. and E.G.-E.), and the Generalitat de Catalunya (ICREA Academia Award to M.C. and 2014SGR-862). The Spanish Ministry of Science is also acknowledged for a Ramýn y Cajal contract to A.C. (RYC-2011-08683)