The electric double-layer capacitances at carbon electrodes with different surface morphologies have been determined in organic solution and polymeric gel electrolytes by an ac impedance method. The capacitance depended not only on the surface structure but also on the electrolyte composition. Activated carbon fiber with pore structure showed higher double-layer capacitance in the polymeric gel than in the solution. The capacitance at glassy carbon with smooth surface was higher in the solution electrolyte than in the gel. The polymeric component in the electrolyte gave different effects on the double-layer capacitance at the carbon electrode depending on the surface morphology. (c) 2006 The Electrochemical Society.