Carbon supported RuSe x (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 RuSe x 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). RuSe x /C (x≅2) with a pyrite structure, produced at 400°C, exhibits catalytic activity and stability superior to that of RuSe x /C with a ruthenium hcp structure or a mixed phase. And this pyrite RuSe x /C (x≅2) catalyst yields H 2 O 2 less than 1.5% in the technically pertinent potential range of 0.4–0.6V (vs. Ag/AgCl). Its stability is verified in 100CV cycles, which shows that the catalyst annealed at 400°C is more enduring in potential cycling over 0.65V (vs. Ag/AgCl), compared with the RuSe x /C catalyst annealed at 300°C and the RuSe cluster /C reference catalyst prepared by thermolysis of Ru 3 (CO) 12 . After CV cycles, the 400°C-annealed catalyst still exhibits a higher ORR activity than the other two catalysts.