In this study we report the pseudocapacitive behavior of multiwalled carbon nanotubes (MWNTs) functionalized with hydrous ruthenium oxide in 1 M sulfuric acid for supercapacitor applications. The entangled network of nanotubes which forms open mesopores and their chemical stability with a basal geometry makes them suitable for such applications. Oxidative treatment to generate oxygenated functional groups on the tube-ends and along the sidewalls enables facile derivatization by hydrous ruthenium oxide to enhance the inherent capacitance. A specific capacitance of 80 F/g is obtained after ruthenium oxide functionalization, which is significantly greater than that of pristine MWNTs (30 F/g) in the same medium. (C) 2003 Elsevier Science B.V. All rights reserved.