We report the release of oxygen (O2) under dark conditions in aerobic soils. This unexpected process is hidden by respiration which constitutes the dominating reversal O2 flux. By using H218O in different soils, we confirmed that 16O18O and 18O2 released under dark soil conditions originated from added H218O. Water is the only large-scale source of electrons for reduction of CO2 in soils, but it has not been considered as an electron donor because of the very strong oxidation system needed. A high share of soil inorganic material seems to favor the release of O2.Keywords: dark water oxidation, soil CO2 reduction, soil oxygen release, soil respiration(Published: 20 December 2013)Citation: Tellus B 2013, 65, 20490, http://dx.doi.org/10.3402/tellusb.v65i0.20490To access the supplementary material to this article, please see Supplementary files under Article Tools online.