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The photo-Fenton activity of graphitic carbon nitride (g-C 3 N 4 ) has been widely studied, nevertheless, its Fenton-like catalytic behavior in the dark has not yet been demonstrated. In the present work, it is shown that oxygenated g-C 3 N 4 obtained at different temperatures (500-600 °C) can degrade indigo carmine with hydrogen peroxide in the dark by a reaction similar to a conventional Fenton's reaction. Based on an extensive characterization of g-C 3 N 4 , we conclude that Fenton-like activity is directly related to the oxygenated functional groups on g-C 3 N 4 structure, mainly by -OH functional groups. Oxygenated functional groups (e.g., hydroquinone-like groups) can reduce the H 2 O 2 and generate oxidizing hydroxyl radicals, just like in the Fenton reaction performed by metals. In addition to new information on g-C 3 N 4 surface reactivity revealed by this study, the metal-free oxygenated g-C 3 N 4 catalyst may be an alternative to traditional metal catalysts used in Fenton-like reactions for advanced oxidation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)