In this work, we propose a method to directly determine the mechanism of the reaction between the nonheme complex FeII(tris(2-pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peracetic acid (AcOOH) in CH3CN, working at room temperature. A multivariate analysis is applied to the time-resolved coupled energy-dispersive X-ray absorption spectroscopy (EDXAS) reaction data, from which a set of spectral and concentration profiles for the reaction key species is derived. These "pure" extracted EDXAS spectra are then quantitatively characterized by full multiple scattering (MS) calculations. As a result, structural information for the elusive reaction intermediates [FeIII(TPA)(κ2-OOAc)]2+ and [FeIV(TPA)(O)(X)]+/2+ is obtained, and it is suggested that X = AcO- in opposition to X = CH3CN. The employed strategy is promising both for the spectroscopic characterization of reaction intermediates that are labile or silent to the conventional spectroscopic techniques, as well as for the mechanistic understanding of complex redox reactions involving organic substrates.