A series of composite electrocatalysts composed of RuO2 nanosheets and carbon supported Pt (RuO(2)ns-Pt/C) has been synthesized by mixing different amounts of commercial 50 wt% Pt/C with RuO(2)ns derived from exfoliation of layered K0.2RuO2.1 center dot nH(2)O. The oxygen reduction activity and stability of the different electrocatalysts has been evaluated as a function of the nanosheet content in the composite electrocatalysts. An increase in initial activity was observed for composite electrocatalyst with RuO2/Pt<0.4. After conducting accelerated durability tests, the RuO(2)ns-Pt/C composite electrocatalyst with RuO2/Pt = 0.3 exhibited a 25% higher mass activity toward oxygen reduction than the pristine Pt/C electrocatalyst. (C) 2014 The Electrochemical Society. All rights reserved.