Aqueous Fe(Ⅳ) is a crucial oxidant in iron-mediated oxidation processes relevant to water purification and atmospheric aqueous systems; yet, its chemical behavior under environmentally relevant pH conditions remains poorly understood. This study elucidated the pH-dependent kinetics and mechanisms of Fe(Ⅳ) formation, self-decay, and secondary reactions during Fe(Ⅱ) ozonation at pH 1.0–5.0. The Fe(Ⅱ)–O₃ reaction involved Fe^Ⅱ(H₂O)₆²⁺ and (H₂O)₅FeII(OH)⁺, generating Fe(Ⅳ) and HO•, respectively; thus, HO• formation increased with increasing pH at pH > 4.0. The determined pKa of Fe(Ⅳ) (Fe^ⅣO²⁺(OH)FeIVO)⁺ was 3.4, primarily modulating its pH-dependent reactivity. Fe(Ⅳ) underwent unimolecular self-decay via Fe^ⅣO²⁺ (k = 0.07 s^-1) and (OH)Fe^ⅣO⁺ (k = 9.7 s^-1), yielding Fe(Ⅲ) and O₂, while its bimolecular decay via (OH)Fe^ⅣO⁺ (k = 8.1 × 10⁴ M^-1s^-1) produced Fe(Ⅲ) and H₂O_2. The Fe(Ⅳ)–Fe(Ⅱ) reaction exhibited marked sensitivity to pH and ionic strength, driven by electrostatic interactions. Fe(Ⅳ) reacted with H₂O₂ via FeIVO²⁺ (k = 2.4 × 10⁴ M^-1s^-1) and (OH)Fe^ⅣO⁺ (k = 6.4 × 10⁴ M^-1s^-1), forming Fe(Ⅲ) and Fe(Ⅱ), respectively. A kinetic model incorporating these reactions accurately predicted Fe(Ⅳ) pH-dependent behaviors and determined the Fe(IV) reaction kinetics with methyl phenyl sulfoxide. These findings significantly advance our understanding of the pH-dependent fate of Fe(Ⅳ) in iron-based oxidation processes.