a High-voltage aqueous electrolyte based supercapacitors (U 4 1.23 V) attract significant attention for next-generation high power, low cost and environmentally friendly energy storage applications. Cell ageing is however markedly pronounced at elevated voltages and results in accelerated overall performance fade and increased safety concerns. Online electrochemical mass spectrometry, combined with cell pressure analysis, is for the first time shown to provide a powerful means for in situ investigation of degradation mechanisms in aqueous electrolyte/carbon based supercapacitors. The activated carbon electrodes possess high specific surface area and oxygen-based surface functionalities (mainly phenol, lactone and anhydride groups), which are oxidized already at a cell voltage of 0.6 V to provoke the evolution of minor amounts of CO and CO 2 . Noticeable water decomposition starts at a high voltage of 1.6 V with the evolution of H 2 on the negative electrode and carbon corrosion on the positive electrode with the generation of predominantly CO. In this paper we also report that short-term cycling leads to partly reversible gas evolution/consumption side-reactions giving negligible capacitance.On the other hand, long-term cycling causes irreversible side-reactions, deteriorates the electrochemical performance, and increases the internal pressure of the cell. Repeated cycling (U o 2 V) is confirmed as a more harmful technique for the electrode integrity compared to the voltage holding in a floating test.In situ gas analysis is shown to provide valuable insights into the electrochemical cell ageing aspects, such as the nature and potential onsets of side-reactions, hence paving the way for fundamental understanding and mitigating the performance and safety loss of high-energy aqueous supercapacitors. Broader contextGlobal energy consumption and environmental concerns steadily increase with the growth of the world's population and the technical development of our society. Future sustainability is largely dependent upon the widespread implementation of renewable energy generation and storage. Supercapacitors constitute a crucial part in the effective energy storage and delivery, e.g. in electric vehicles (start-stop systems) and wind mills (balancing the energy provided to the grid), because of their exceptional power capability and long cycle life. Supercapacitors with high surface area carbon electrodes operating in aqueous electrolytes offer high power rates, low cost, safe and environmental benign devices, but at moderate energy densities. Significant energy enhancement can be achieved by applying higher operating cell voltages (U 4 1.23 V) when using sulphate-based electrolytes. However, for practical implementations a fundamental understanding of their premature ageing as well as related safety issues are required. In the present work, state-of-the-art in situ gas analysis is shown to deliver valuable insights into the governing mechanisms behind the ageing of these devices. Assignment of safe operating voltage ...