Oxidation of Orange II ([4-[(2-hydroxynaphtyl)azo]benzenesulfonic acid], sodium salt) by hydrogen peroxide catalyzed by iron(III) complexed to tetra amido macrocyclic ligands (Fe^III–TAML activators) in aqueous solutions at pH 9–11 leads to CO₂, CO, phthalic acid and smaller aliphatic carboxylic acids as major mineralization products. The products are non-toxic according to the Daphnia magna test. Several organic intermediates have been identified by HPLC and GC-MS that allowed the detailed description of Orange II degradation. The catalytic oxidation can also be performed by organic oxidants such as benzoyl peroxide, tert-butyl and cumyl hydroperoxides. Kinetic studies of the catalyzed oxidation indicated that Fe^III–TAML activators react first with ROOR′ to form an oxidized catalyst (k_I), which then oxidizes Orange II (k_II). Neglecting the reversibility of the first step, the rate equation is rate = k_Ik_II[Fe^III][ROOR′][Dye]/(k_I[ROOR′] + k_II[Dye]); here Fe^III and ROOR′ represent the catalyst and peroxide, respectively. The rate constant k_I equals (74 ± 3) × 10³, (1.4 ± 0.1) × 10³, 24 ± 2, and 11 ± 1 M⁻¹ s⁻¹ for benzoyl peroxide, H₂O₂, t-BuOOH, and cumyl hydroperoxide at pH 9 and 25 °C, respectively. An average value of k_II equals (3.1 ± 0.9) × 10⁴ M⁻¹ s⁻¹ under the same conditions. The unraveling of the kinetic mechanism allows the comprehension of the robust reactivity, and this is discussed in detail using the representative results of DFT calculations.