Dark formation of hydroxyl radical upon oxidation of reduced iron minerals plays an important role in the degradation and transformation of organic and inorganic pollutants. Herein, we compared the hydroxyl radical formation from various reduced iron minerals at different redox conditions. ·OH production was generally observed from the oxidation of reduced iron minerals, following the order: mackinawite (FeS) > reduced nontronite (iron-bearing smectite clay) > pyrite (FeS2) > siderite (FeCO3). Structural Fe2+ and dissolved O2 play critical roles in ·OH production from reduced iron minerals. ·OH production increases with decreasing pH, and Cl- has little effect on this process. More importantly, dissolved organic matter significantly enhances ·OH production, especially under O2 purging, highlighting the importance of this process in ambient environments. This sunlight-independent pathway in which ·OH forms during oxidation of reduced iron minerals is helpful for understanding the degradation and transformation of various inorganic and organic pollutants in the redox-fluctuation environments.