The utilization of hydrous ruthenium oxide (denoted as RuOx.nH(2)O) was promoted by annealing the oxide in air as well as by mixing it with conductive activated carbon (AC) due to the significant improvement in intra- and interparticle electronic conductivity, respectively. The maximum specific capacitance (C-S,C-RuOx) of RuOx.OnH(2)O, 1340 F/g (measured at 25 mV/s), very close to the theoretic value, was obtained from a composite consisting of AC and RuOx.nH(2)O coated on graphite (denoted as AC-RuOx/G) with 10 wt % of sol-gel-derived RuOx.nH(2)O nanodots annealed in air at 200 degreesC for 2 h. The UV absorption spectral features showed a shift in lambda(max) to the red as the mean particle size of RuOx.nH(2)O nanodots was increased, attributable to the surface plasmon resonance phenomenon. The average particle size of highly uniform RuOx.nH(2)O nanodots, ranged from 2.05 to 3.01 nm, was estimated from the high-resolution transmission electron microscopy. The dependence of capacitive performance on the size and content of RuOx.nH(2)O nanodots, evidenced by cyclic voltammetry and electrical impedance spectroscopy results, revealed the important influences of interparticle electronic conductivities on the utilization of RuOx.nH(2)O. The RuOx.nH(2)O nanodots with and without annealing in air at 200 degreesC for 2 h showed the amorphous structure from both the X-ray diffraction and electron diffraction analysis. (C) 2004 The Electrochemical Society.