Increasing the available cell voltage for electrochemical double layer capacitors (EDLC) is one route to simultaneously increase energy density and power density of the EDLC. Increased cell voltage may. however, introduce faradaic reactions such as ion insertion and electrolyte decomposition, which potentially limit the lifetime of the device. Using a purpose designed pressure cell, we have, for the first time, measured the pressure increase in capacitor cells based oil real EDLC electrode coils in 1 M (C2H5)(4)NBF4/propylene carbonate electrolyte during cycling between 0 and 2.5 V and for constant cell voltages LIP to 3 V. During cycling a reversible pressure decrease was observed upon charging. All irreversible pressure increase was monitored during load tests at constant cell voltages above 2.5 V. The absolute amount of gases evolved could be determined by means of simultaneous compressibility monitoring. (c) 2006 Elsevier Ltd. All rights reserved.