Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India E-mail: rajaraman@chem.iitb.ac.in Manuscript received online 22 April 2019, revised and accepted 15 May 2019 High-valent iron(IV/V)-oxo species are known as the potent oxidant in various organic transformations such as C-C, C-H activation reactions. While the aggressive oxidative abilities of these species are established, very little is known about the oxidative abilities of the precursor complexes that generate these species. This is particularly important, as often these species are not generated with 100% conversion and present a possibility with the coexistence of the catalytic precursor in the solution. Given this background, it is important to establish the comparative oxidative ability of the precursor materials viz a viz iron(IV/V)-oxo species. In our current study, along with high-valent iron(IV/V)-oxo species, we have also tested the reactivity of the iron(III)-iodosylarene towards epoxidation reaction. DFT methods have been taken into account to establish a detailed mechanism of this reaction. A comprehensive study of the mechanism of epoxidation of styrene reveals that the energetic requirement of generating the iron(IV/V)-oxo species from the iron(III)-iodosylarene is on par with the direct olefin epoxidation by the iron(III)-iodosylarene. As there are additional barriers for the high-valent oxo species to activate olefin, it is safe to conclude iron(III)-iodosylarene as the potent oxidant in this reaction. A careful electronic structure analysis reveals that ligand design is playing a major role in tuning the reactivity of the iron(III)-species hence altering the entire landscape of the catalytic transformations. This work unequivocally establishes that Fe(III)-iodosylarene as a powerful oxidant. Since this act as a key catalytic precursor for the formation of FeIV=O species, this likely to trigger an intense debate on the need to evoke aggressive oxidant such as FeIV=O in other catalytic transformations.