A reliable method for the determination of regions of electrochemical stability of electrolytes for electrochemical double layer capacitors (EDLC) by means of potential window opening measurements is presented. For this purpose, a new evaluation method for the anodic and cathodic potential limits of electrochemical systems utilizing high surface area carbon electrodes is suggested. A change of 5% in the faradaic current contribution within a potential step of 1 V to the overall current is proposed as new stability criterion. The results of seven ionic liquids (IL) and three standard electrolytes dissolved in acetonitrile (AN) or propylene carbonate (PC) at RT are presented. For two electrolytes a temperature dependent study is conducted. The feasibility of the new criterion is verified by full cell constant voltage aging experiments. The IL [EMIMI[BF4] displays a maximum cell voltage of 3.5 Vat RT, which is, compared to standard electrolytes, an increase of about 0.5 V. (C) 2013 Elsevier Ltd. All rights reserved.