Phenolic resin-based activated carbon fibers (ACFs) were prepared by KOH, CO2, steam, Zr(acac)(4), and ferrocene activation. The porosity, surface chemistry, and electric conductivity of ACFs were studied as electric double-layer capacitors in KOH electrolyte. The specific capacitance for ACFs is generally in the range 130-180 F g(-1) and roughly has a linear relationship with the Brunauer, Emmett, and Teller (BET) surface area (S-BET). Among all of the ACFs, ACF-KOH-200 (which has a SBET of only similar to 1284 m(2) g(-1) and an average pore diameter of 14.3 (A) over circle, but has the largest amount of hydroxyl carbonyl and quinone groups on the surface ) exhibits the maximum value of 215 F g-1, indicating that although the capacitance is controlled basically by the S-BET, the oxygen groups play a dominant role in the formation of the double layers. The presence of mesopores in ACFs with very high S-BET and good electric conductivity is not effective for double-layer capacitors. The number of ions participating in the double layers per surface area unit increases with the decrease of the pore size in the range of 11-20 (A) over circle; beyond 20 (A) over circle, it is nearly constant. No simple correlation was found between the resistance and the capacitance. (c) 2007 The Electrochemical Society.