Amorphous RuO2·xH2O and a VGCF/RuO2·xH2O nanocomposite (VGCF=vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0V at various scan rates and with an electrolyte solution of 1.0M H2SO4. The specific capacitance of RuO2·xH2O and VGCF/RuO2·xH2O nanocomposite electrodes at a scan rate of 10mVs−1 is 410 and 1017Fg−1, respectively, and at 1000mVs−1 are 258 and 824Fg−1, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 104 cycles shows that the RuO2·xH2O and VGCF/RuO2·xH2O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.