The influence of acetylene black (AB) content (0-20 wt.%) on the electrochemical performance of activated carbon based electrochemical double layer capacitor (EDLC) electrodes has been studied systematically. The electrochemical performance was evaluated by galvanostatic charging/discharging. cyclic voltammetry, and alternating current impedance. Experimental results indicated that 5 wt.% AB was the best choice for electrodes presenting high energy density and good rate capability. AB ensures that the composite electrodes present high electronic conductivity: however, scanning electron microscope images show that excessive AB form large agglomerates, which stuff the voids among activated carbon particles and hinder the electrolyte ion transfer during the electrochemical double layer charging/discharging. Thus the composite electrodes with excessive AB lose their hierarchically porous structure and the subsequent ion-sieving effect deteriorates the electrochemical performance. (C) 2008 Elsevier B.V. All rights reserved.