In this work we present a novel on-line electrochemical mass spectrometer design, which enables quantitative gas evolution analysis with a sealed battery design, applied to the study of the charging processes in a Li-O-2 battery. Successive charge/discharge cycles were performed using Vulcan-carbon based positive electrodes in electrolytes composed of 0.2 M LiTFSI and two different solvents: i) propylene carbonate (PC), and, ii) bis(2-methoxyethyl) ether (diglyme). Results on the PC based electrolyte reveal a strong potential dependence of the evolved gaseous products which is maintained throughout subsequent cycles, consisting predominantly of O-2 below 3.7 V and of predominantly CO2 above 3.7 V. The observed capacity fading is most likely caused by the gradual accumulation of discharge products which can only be oxidized at high anodic potentials. With diglyme electrolyte, the predominant gas during charging is O-2. However, while the number of electrons/O-2 closely corresponds to the oxidation of Li2O2 at the beginning of each charging cycle (2 e(-)/O-2), it increases with potential and with the number of cycles, suggesting the gradual formation of other oxygen-containing discharge products which can only be oxidized at high potential with the parallel formation of CO2. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.042303jes] All rights reserved.