Lithium ion batteries containing lithium titanate (LTO), Li4Ti5O12, as anode material are promising energy storage systems for their safety and long cycle-life. However, while there are few studies that demonstrate very long cycle-life of LTO-anode based batteries, a significant number report extensive evolution of hydrogen during cycling or storage of LTO cells ("gassing"), accompanied by strong capacity fading and poor cycling performance. Here, we analyzed LTO cells in-operando with on-line electrochemical mass spectrometry (OEMS) and determined the real-time concentration of evolved gases. With this method, we were able to determine a direct correlation between the amount of evolved hydrogen and the water content of the electrolyte upon charge and discharge. Our analysis suggests that the one-electron reduction reaction of water leads to hydrogen evolution and to the formation of hydroxide anions. The latter trigger a ring-opening reaction of cyclic carbonates and lead to solvent decomposition into CO2 and carbonate oligomers (detected by ex-situ ATR-FTIR). The deposition of the latter on the electrodes is likely responsible for the performance and capacity loss over a longer time period. We propose that the gassing of LTO batteries reported in some studies is caused by water intrusion during cell assembly or by water contained in improperly dried electrodes. (C) 2014 The Electrochemical Society. All rights reserved.