Methyl phenyl sulfoxide (PMSO), conventionally employed to probe high-valent metal-oxo species in persulfate processes, is reveals herein as a substrate-dependent mediator that selectively enhanced/inhibited degradation of various pollutants in Fe(II)/peroxymonosulfate (PMS) systems. Control experiments across transition-metal systems (Fe(II)/Co(II)/Cu(II)/Mn(II)/Fe(III)) confirmed that this phenomenon is unique to Fe(II)-based systems. The system-dependent behavior of PMSO is demonstrated to correlate with ROS generated in each system. In Fe(II)/PMS systems dominated by FeIVO2+, SO4·- and unidentified iron-centered species, PMSO exhibits dual functionality: as an electron shuttle, enhancing degradation of electron-rich pollutants by facilitating electron transfer and promoting the Fe(III)/Fe(II) cycle; conversely, for electron-deficient pollutants, PMSO functions as a conventional scavenger, suppressing degradation. Quantitative structure-activity relationships (QSARs) modeling establishes that PMSO enhances degradation for pollutants with the highest occupied molecular orbital (HOMO) ≥ -8.3154 eV and ionization potential (IP) ≤ 6.9711 eV; otherwise, it inhibits degradation. In Co(II)/PMS systems dominated by Coᴵᴵ-PMS* complexes, PMSO competitively consumes Coᴵᴵ-PMS* and quenches SO4- resulting in uniform inhibition of all pollutants. Negligible PMSO effect was observed in Cu(II)/Mn(II)/Fe(III) systems with ineffective PMS activation. This work challenges the uncritical use of PMSO as an inert probe in Fe(II) systems, highlights metal-peroxo complexes as pivotal intermediates dictating electron-transfer pathways and provides principles for designing selective advanced oxidation processes (AOPs) through mediator tuning.