An expanded graphite (e-MCMB, mesocarbon microbeads) having a wider interlayer spacing (d(002)=0.404 nm) than that of common graphites is prepared by heat-treatment of an oxidized MCMB. When the e-MCMB electrode, which gives a negligible capacitance due to a small surface area, is polarized over a certain onset potential [4.6-4.8 V (vs Li/Li+) for positive and 1.3-1.0 V for negative direction], it is electrochemically activated to be a high-capacitance positive and negative electrode for electrochemical capacitor. The activation process involves an ion intercalation into the interlayer space to generate ion-accessible sites. The intercalation is evidenced by the presence of a voltage plateau in the charge-discharge profiles, and by the widening of the interlayer distance (by in situ X-ray diffraction study) and concomitant electrode swelling (by electrochemical dilatometry) that occur at the same potential region. The electrochemically activated e-MCMB particles carry slitlike pores of ca. 0.45 nm in the mean interlayer distance, into which ions very likely enter either bare or with partial solvent shells with a mixed adsorption/intercalation charge storage behavior. A full cell fabricated with two e-MCMB electrodes delivers a volume specific capacitance of 30-24 F mL(-1) within 100 cycles for a dry electrode pair at a working voltage of 3.7 V. (C) 2008 The Electrochemical Society.