Carbon nanotube sheet electrodes have been prepared from catalytically grown carbon nanotubes of high purity and narrow diameter distribution, centered around 80 Å. Our study shows that the electrodes are free-standing mats of entangled nanotubes with an open porous structure almost impossible to obtain with activated carbon or carbon fiber. These properties are highly desirable for high power and long cycle life electrochemical capacitors. Specific capacitances of 102 and 49 F/g were measured at 1 and 100 Hz, respectively, on a single cell device with 38 wt &percent;H2SO4as the electrolyte. The same cell had a power density of>8000W/kg. ©1997 American Institute of Physics.