Reducing the particle size is a straightforward way to increase the specific surface area of ruthenium oxide, which usually translates to the high specific capacitance for a supercapacitor. Herein, we report a facile strategy to fabricate ultrafine ruthenium oxides supported on various carbon-based substrates (carbon powders, carbon nanotubes, or reduced graphene oxides) as excellent electrode materials for a supercapacitor. The novelty of this work lies in its synthetic approach, which involves an aqueous synthesis of ruthenium nanoclusters under the control of pH value, and an air oxidation-based conversion process. In particular, owing to their ultrafine particle size, the as-prepared carbon-, carbon nanotube-, or reduced graphene oxide-supported ruthenium oxides exhibit specific capacitance as high as 879.1 F g(-1), 966.8 F g(-1) and 1099.6 F g(-1), respectively, for a supercapacitor at a current density of 0.5 A g(-1). The specific capacitance maintains 98.4% (for carbon supports), 98.0% (for carbon nanotube supports) and 98.4% (for reduced graphene oxide supports) at current density of 1 A g(-1) with good cycling stability. The remarkable simplicity and environmental friendliness of this synthesis may provide a liable quantity production route to produce ruthenium oxides as highly efficient electrode materials for a supercapacitor. (C) 2016 Elsevier Ltd. All rights reserved.