The Ru85Se15 nanoparticles supported on commercial Vulcan XC-72R or multi-walled carbon nanotubes (MWCNTs) were synthesized by microwave assisted polyol method with different solution pH. The Ru85Se15 nanoparticles on citric acid (CA)-treated supports prepared at pH = 7 exhibited the most uniform particle distribution, higher degree of graphitization on supports, and four-electron ORR mechanism. Using lower loading of 0.138 mg Ru cm(-2), the maximum power densities (P-max) for the Ru85Se15/CA-MWCNTs and Ru85Se15/CA-XC72R were 380 mW cm(-2) at 1430 mA cm(-2) and 336 mW cm(-2) at 1230 mA cm(-2), respectively, with oxygen, while 166 mW cm(-2) at 710 mA cm(-2) and 126 mW cm(-2) at 510 mA cm(-2), respectively, with air. The P-max of 103 mW cm(-2) with air for the Ru85Se15/CA-MWCNTs could be retained (38% loss), while 46 mW cm(-2) for Ru85Se15/CA-XC72R (64% loss) upon 6000 cycles. The four-electron ORR mechanism and highly graphitized MWCNTs might be responsible for the high performance and durability of Ru85Se15/CA-MWCNTs. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.