Carbon supported RuSex (x = 0.35-2) catalysts of controlled stoichiometry and phase are synthesized via precipitating ruthenium nanoparticles on Vulcan XC-72R, then selenizing ruthenium with hydrogen annealing. The competition for Se between solid-state Ru selenization and volatile selenium hydride formation results in RuSex nanoparticles with the pyrite structure, the Ru hcp structure, or a mixture of the two. These catalysts are methanol tolerant, and catalytically active toward oxygen reduction reaction (ORR). RuSex/C (x congruent to 2) with a pyrite structure, produced at 400 degrees C, exhibits catalytic activity and stability superior to that of RuSex/C with a ruthenium hcp structure or a mixed phase. And this pyrite RuSex/C (x congruent to 2) catalyst yields H2O2 less than 1.5% in the technically pertinent potential range of 0.4-0.6 V (vs. Ag/AgCl). Its stability is verified in 100 CV cycles, which shows that the catalyst annealed at 400 degrees C is more enduring in potential cycling over 0.65 V (vs. Ag/AgCl), compared with the RuSex/C catalyst annealed at 300 degrees C and the RuSecluster/C reference catalyst prepared by thermolysis of Ru-3(CO)(12). After CV cycles, the 400 degrees C-annealed catalyst still exhibits a higher ORR activity than the other two catalysts. (C) 2009 Elsevier Ltd. All rights reserved.