The capacitive behaviors of activated carbon (AC) were investigated in K4Fe(CN)(6). It was revealed that the AC electrodes in K4Fe(CN)(6) exhibited high capacitance (183, 232 and 272 F.g(-1) in a cell voltage window of 0-0.8, 0-1.0 and 0-1.2 V, respectively) due to the contribution of the double layer capacitance from AC and the introduction of pseudocapacitance by the redox reactions of Fe(CN)(6)(4-)/Fe(CN)(6)(3-). In a symmetric AC capacitor, the negative electrode shows the characteristics of a pure capacitive behavior, but the positive electrode is characterized by pseudocapacitive behavior, which means that the redox process occurs on the positive electrode. The mechanism of the charge storage processes for AC in K4Fe(CN)(6) suggests AC has played an important role for Fe(CN)(6)(4-) adsorption and thus improving electron-transfer process compared with the bare Ni electrode. These results demonstrate the potentialities of using electrolyte of K4Fe(CN)(6) for producing double layer capacitance and pseudocapacitance simultaneously in a symmetric activated carbon supercapacitor. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3591061] All rights reserved.