In this study, a Fenton-activated persulfate (Fe 2+ /PS) system was introduced for the efficient degradation of Mordant Blue 9 (MB 9) as a textile dye in an aqueous solution. Results showed that the degradation of MB 9 was markedly influenced by operational parameters, such as initial pH, PS concentration, Fe 2+ concentration, and initial dye concentration. Optimal reaction conditions were then determined. Inorganic anions, such as Cl − and HCO 3 − , enhanced the degradation efficiency of MB 9 under optimal conditions. Addition of HCO 3 − reduced the degradation performance of MB 9, whereas the addition of Cl − increased the degradation percentage of MB 9. In addition, quenching experiments were conducted using methanol and tert-butyl alcohol as scavengers, and methanol was identified as an effective scavenger. Thus, the degradation of MB 9 was attributed to SO •− 4 and • OH radicals. The degradation and mineralization efficiency of MB 9 was significantly reduced using the conventional Fenton process i.e., Fe 2+ / hydrogen peroxide (HP) because of the formation of a Fe complex during degradation. Meanwhile, the Fe 2+ /persulfate (PS) system improved the degradation and mineralization performance.