The formation of halogenated disinfection byproducts (DBPs) during organic pollutants degradation in advanced oxidation processes (AOPs) has raised growing concerns globally. However, the detailed roles of chlorine ions (Cl-) in pollutant degradation by peroxymonosulfate (PMS) alone remain controversial so far. Here, we shed light on the PMS-Cl--phenol interactions and revealed a previously overlooked pH-dependent phenol transformation pathway. At acidic pH, Cl- significantly accelerated the phenol removal. The Cl--PMS reaction resulted in abundant reactive chlorine species, which attacked phenol to form 4-chlorophenol and 2, 4-di-chlorophenol as the main transformation products. In contrast, the impact of Cl-became negligible at alkaline pH. Raising the pH severely suppressed the phenol chlorination and significantly stimulated generation of singlet oxygen (O-1(2)) for degradation of phenol and the minor formed chlorophenols. The O-1(2) was resulted from a combined effect of PMS self-decomposition and PMS-benzoquinone interaction at alkaline pH. Our results imply a previously-underestimated impact of Cl- on phenolic compounds degradation in PMS-based AOPs and underscore a potential risk of chlorinated DBPs formation under environmental conditions.