In this work, carbonaceous materials were modified in order to improve capacitance and charge density in electric double layer capacitors (EDLCs). Optimal conditions for plasma surface treatment of activated carbon have been examined for times from 10 min to 1 h at 150 degrees C. The plasma is a high-frequency glow discharge in N-2. The pressure of the gas is 13.3 Pa. The electrode is set up so that the EDLC sample is covered with the glow discharge. Space charge density can be improved by plasma surface treatment of carbon materials. Also, space charge distributions of EDLC after the plasma surface treatment have been examined for temperatures from 20 to 80 degrees C. (C) 2007 Elsevier B.V. All rights reserved.