A novel method based on spontaneous reduction of Ru(VI) and Ru(VII) is reported for the deposition of Ru oxide on multi-walled carbon nanotubes (MWCNT). Both purified and acid functionalized nanotubes (p-MWCNT and a-MWCNT) have been used to produce composite materials for use in high power aqueous supercapacitors. Specific capacitances of 213±16Fg−1 and 184±11Fg−1 were obtained for Ru oxide/p-MWCNT and Ru oxide/a-MWCNT composites, respectively. Specific capacitances for the Ru oxide component were 704±62Fg−1 and 803±72Fg−1, respectively. Current vs. potential curves exhibited capacitance peaks at ca. +0.5V vs. Ag/AgCl. The Ru oxide/p-MWCNT composite was shown to be stable over 20,000 charge/discharge cycles. An advantage of the method is that no pre-treatment of the MWCNT is required for optimum performance.