This Letter describes a simple, aqueous-phase route to the synthesis of ultrasmall ruthenium oxide (RuO2) nanocrystals with sizes less than 2 nm. These ultrasmall RuO2 nanocrystals can be synthesized simply by heating an aqueous solution containing RuCl3 in the presence of Vulcan carbon powders as supports. As a result of exceptionally small particle sizes, the carbon-supported RuO2 nanocrystals exhibited a specific capacitance as high as 517.8 F/g based on the mass of RuO2. Our approach provides a simple, environmentally benign, and readily scalable route to the synthesis of RuO2 nanocrystals that are promising candidates as electrode materials for high-performance supercapacitors. (C) 2013 Elsevier B.V. All rights reserved.