To quantitatively probe iron intermediate species [Fe­(V)/Fe­(IV)] in Fe­(VI) oxidation, this study systematically investigated the reaction kinetics of Fe­(VI) oxidation of 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonic)­acid (ABTS) at different ratios of [ABTS]0/[Fe­(VI)]0 (i.e., >1.0, =1.0, and <1.0) in pH 7.0 phosphate (10 mM)-buffered solution. Compared to the literature, a more comprehensive and robust kinetic model for the Fe­(VI)–ABTS system including interactions between high-valent iron species [Fe­(VI), Fe­(V), and Fe­(IV)], ABTS, and the ABTS•+ radical was proposed and validated. The oxidation of ABTS by Fe­(VI) (k = (5.96 ± 0.9%) × 105 M–1 s–1), Fe­(V) (k = (2.04 ± 0.0%) × 105 M–1 s–1), or Fe­(IV) (k = (4.64 ± 13.0%) × 105 M–1 s–1) proceeds via one-electron transfer to generate ABTS•+, which is subsequently oxidized by Fe­(VI) (k = (8.5 ± 0.0%) × 102 M–1 s–1), Fe­(V) (k = (1.0 ± 40.0%) × 105 M–1 s–1), or Fe­(IV) (k = (1.9 ± 17.0%) × 103 M–1 s–1), respectively, via two-electron (oxygen atom) transfer to generate colorless ABTSox. At [ABTS]0/[Fe­(VI)]0 > 1.0, experimental data and model simulation both indicated that the reaction stoichiometric ratio of Fe­(VI)/ABTS•+ increased from 1.0:1.0 to 1.0:1.2 as [ABTS]0 was increased. Furthermore, the Fe­(VI)–ABTS–substrate model was developed to successfully determine reactivity of Fe­(V) to different substrates (k = (0.7–1.42) × 106 M–1 s–1). Overall, the improved Fe­(VI)–ABTS kinetic model provides a useful tool to quantitatively probe Fe­(V)/Fe­(IV) behaviors in Fe­(VI) oxidation and gains new fundamental insights.