Results obtained for polyacrylonitrile microcellular foam thin films deposited on carbon fibers are presented. It appears that surface modifications by CO2 flow or water steam ar high temperature lead to an increase of the film surface area, as well as a proportional increase of capacitance. The films were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy. and the Brunauer-Emmett-Teller technique. Extensive electrochemical studies (cyclic voltnmmetry, ae impedance, galvanic cycles, and cycle life tests) were performed both in aqueous and organic media in order to evaluate their use ax electrodes in a supercapacitor. The importance of the nature of the electrolyte was demonstrated using lithium perchlorate and tetraalkyl-based salts. The high performances of these activated materials were correlated both to their high surface area and their microporous distribution. (C) 2000 The Electrochemical Society. S0013-4651(00)04-096-9. All rights reserved.