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Oxygen vacancies (OVs)-rich α-Ni 0.2 Fe 1.8 O 3 with dual metal sites was synthesized by Ni2+ isomorphously substituted α-Fe 2 O 3 to activate peroxymonosulfate (PMS) for aceclofenac (ACF) degradation. A novel insight was proposed for the overlooked role of H 2 O in the surface-complexed process between PMS and unsaturated metal sites induced by OVs. The surface-bond SO 4 ∙- and ∙OH were demonstrated as the dominant reactive oxygen species (ROS). In situ ATR and Raman spectra revealed the reaction of HSO 5 - with the surface metal sites by replacing the surface-bonded -OH. The DFT study revealed the spontaneous formation of hydrolyzed -OH on OVs (∆G=−1.45 eV), which was further replaced by HSO 5 - to complex with metal sites (∆G=−3.58 eV). The O Ⅰ of the H-O Ⅰ -O Ⅱ -SO 3 - was more readily bonded with the Ni or Fe sites around OVs, causing the O-O cleavage for SO 4 ∙- generation. This study proposed a brand-new perspective for the interfacial behavior of PMS activation. [Display omitted] • The α-Ni 0.2 Fe 1.8 O 3 with rich oxygen vacancies can excellently activate PMS. • The surface-bond SO 4 ∙- and ∙OH played a major role in the degradation of ACF. • HSO 5 - was mainly bonded to metal sites by replacing surface -OH groups. • The interfacial mechanism for PMS activation by α-Ni 0.2 Fe 1.8 O 3 was proposed. • The degradation pathway of ACF by α-Ni 0.2 Fe 1.8 O 3 /PMS system was deduced. [ABSTRACT FROM AUTHOR]