Mesoporous activated carbon with high surface area (AC-Y-A) was prepared by carbonization followed by activation of vinylidene chloride copolymer containing yttrium acetylacetonate. Their performances as electrodes for electric double layer capacitor (EDLC) in tetraalkylammonium tetrafluoroborate (R4NBF4)/propylene carbonate (PC) or acetonitrile (AN) solutions were evaluated and compared with those of microporous activated carbons [AC and AC(CO2)]. The specific EDLC capacitances of the mesoporous AC-Y-A are almost the same as those of microporous AC at low discharge current. However, at high discharge current, the specific capacitance of AC-Y-A is much higher than those of both AC and AC(CO2) in PC and AN solutions. The capacitances decreased with increasing alkyl chain length of R-4 N+ ions and discharge current. These results suggest that EDLC capacitance depends on pore size of activated carbons and smooth movement of R-4 N+ ions in pores. (c) 2005 Springer Science + Business Media, Inc.