Complex capacitance analysis was done on the porous carbon electrode-electrolyte interface, where a minor leakage current is involved in addition to the dominant capacitor charging current. Based on the transmission line model, imaginary capacitance profiles (C-im vs. log f) were theoretically derived for a cylindrical pore and multiple pore systems of nonuniform pore geometry. The parallel RC circuit was assumed for the interfacial impedance, where R is the charge-transfer resistance for leakage current and C the double-layer capacitance. The theoretical derivation illustrated that the resistive tail relevant to the leakage current appears in addition to the capacitive peak, which was in accordance with the experimental data taken on the porous carbon electrode. The electric double-layer capacitor (EDLC) parameters such as the extent of leakage current, total capacitance, and rate capability were visually estimated from the imaginary capacitance profiles. The more quantitative EDLC parameters were obtained by a nonlinear fitting to the experimental data. (c) 2005 The Electrochemical Society. All rights reserved.