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Venlafaxine, a representative antidepressant, has been detected frequently in aquatic environments. The treatment of venlafaxine by free chlorine (NaOCl) and chlorine dioxide (ClO 2) was investigated in this study. The effects of operational variables and the water matrix on venlafaxine degradation were evaluated. The transformation pathways of venlafaxine were also studied. The results indicated that venlafaxine was removed efficiently during disinfection processes, especially when reacted with ClO 2. A higher dosage of disinfectant and mildly alkaline conditions (pH 9) enhanced the degradation of venlafaxine. The reactions were impacted when the tests were conducted in real water matrices, especially in secondary effluent. The presence of chloride and low concentrations of fulvic acid enhanced venlafaxine decomposition. The presence of Br− also accelerated the reaction between venlafaxine and NaOCl. However, NO 2 − inhibited venlafaxine removal in both disinfection processes. Six intermediates were identified during venlafaxine degradation by ultrahigh-performance liquid chromatography with quadrupole-time-of-flight mass spectrometry, and the main reactions included dehydration and demethylation. [Display omitted] • Venlafaxine was removed efficiently both by NaOCl and ClO 2. • Venlafaxine chlorination fits pseudo-first-order reaction kinetic. • Low concentration of fulvic acid and Cl− improved venlafaxine removal. • Main reaction mechanism included dehydration and demethylation. • ClO 2 is a proper disinfectant for venlafaxine decomposing. [ABSTRACT FROM AUTHOR]