Composite electrodes which comprise a non-conductive activated carbon of large surface area (1420 m(2) g-L) and a conductive carbon black (CB) of small surface area (220 m2 g(-1)) have been prepared and studied for their capacitive properties in aqueous KOH and Na2SO4 electrolytes. For either electrolyte, maximum capacitance exists at the composition believed to correspond to the percolation threshold for CB, the conductive phase. At a CB content less than the threshold, the capacitance is limited mainly by the electronic resistance on the electrode side. The interfacial surface area becomes the limiting factor as the threshold is exceeded. A maximum capacitance of 108 F g(-1) at a voltage sweep rate of 20 mV s(-1) is obtained in 1 M KOH aqueous electrolyte with a CB content of 25 wt.% (or similar to14 vol.%).