Aprotic Li-O-2 cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe low discharge capacities and formation of passivating layers of Li2O2 on the electronically conducting cathode support, while other groups report large capacities and toroidal Li2O2 crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O-2 cells, having a large effect on discharge capacity and Li2O2 morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li2O2 is still the main discharge product in Li-O-2 cells containing water, and moreover the Li2O2 yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.