Porous carbons (Brunauer-Emmett-Teller surface area 999 m(2)/g) prepared by defluorination of polytetrafluoroethylene (PTFE) powder with potassium metal showed a very high capacitance of the electrical double layer (229 F/g). Such a high value is considered to be due not only to the contribution of microporosity, but also to the development of mesoporosity. When the original PTFE was subjected to gamma-ray irradiation to reduce the polymerization degree, the carbon obtained in the same procedure showed only a slightly higher value (237 F/g). The relationship between the capacitance and pore size was discussed by taking into consideration the results for conventional activated carbon fibers. (C) 2002 The Electrochemical Society.