The activated carbon in electrochemical double layer electrodes undergoes slow ageing. The effect becomes only apparent when capacitors are polarized to voltages at or above specified limits, or at high temperatures, for at least several months. A combination of “post mortem” analysis methods shows that the pores in the carbon are modified structurally, mainly by clogging up with organic species, but also electrochemically by decomposition of electrolyte, and by reduction and oxidation of the carbon in the electrodes. Key chemicals are trace water, the solvent acetonitrile, and the salt tetraethyl ammonium tetrafluoroborate. The changes differ hugely between negative and positive pole; the positively polarized electrode is much more affected. Also other compounds of the device, e.g. aluminum electrode support and paper separator, exhibit chemical ageing.