Laboratory-scale electrochemical capacitor cells with bound activated carbon electrodes and acetonitrile-based electrolyte were aged at various elevated constant cell voltages between 2.75V and 4.0V. During the constant voltage tests, the cell capacitance as well as the capacitance and resistance of each electrode was determined. Following each aging experiment, the cells were analyzed by means of electrochemical impedance spectroscopy, and the individual electrodes were characterized by gas adsorption and X-ray photoelectron spectroscopy. At cell voltages above 3.0V, the positive electrode ages much faster than the negative. Both the capacitance loss and resistance increase of the cell could be totally attributed to the positive electrode. At cell voltages above 3.5V also the negative electrode aged significantly. X-ray photoelectron spectroscopy indicated the presence of degradation products on the electrode surface with a much thicker layer on the positive electrode. Simultaneously, a significant decrease in electrode porosity could be detected by gas adsorption.