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Sulfate ions (SO 4 2- ) are ubiquitous in aqueous environments and are generally considered to be inert due to their chemical stability. For the first time, we found that SO 4 2- can be activated into SO 4 2- -type radicals (e.g., SO 3 - , SO 5 - , SO 4 - ) in the presence of phenolic compounds under simulated or natural solar irradiation. In turn, the radicals promoted the transformation and mineralization of phenolic compounds compared to that in the absence of SO 4 2- with reaction rate constants ranging from 0.008 h -1 to 0.021 h -1 . In addition, the activation mechanisms of inert SO 4 2- in the photochemical transformation of phenolic compounds were elucidated. A hydrated electron (e aq - ) is first generated during the photolysis of phenolic compounds and is the most important step in the activation of SO 4 2- . Then, the e aq - reduces SO 4 2- to SO 3 2- , and SO 3 2- is further photochemical activated to become a reactive species (e.g., e aq - and SO 3 - ), which can evolve into strong oxidants (e.g., SO 5 - and SO 4 - ), via a series of radical chain reactions. These oxidants are responsible for the enhanced phenolic compound degradation. The photochemical activation of seemingly inert SO 4 2- sheds new light on studies on the transport, transformation and environmental impact of matter (e.g., phenolic compounds) in specific water environments and provides a novel strategy for the generation of SO 4 - and photochemical removal of phenolic pollutants., (Copyright © 2018 Elsevier Ltd. All rights reserved.)