Various kinds of activated carbon/activated carbon fibers were used in the evaluation of electrical double layer capacitors using the method of image analysis. The appropriate hydrated ion structures in an aqueous system of H2SO4 /H2O and an organic system of LiClO4/Polypropylene carbonate were calculated using the software Cerius(2) (ver. 3.8). The capacitance obtained varied with the electrolyte used, even though the capacitor material remained the same. The relationship between the pore size and the electrolyte ion diameter is discussed.