Oxygen reduction reaction (ORR) was carried out in a sulfuric acid solution saturated with oxygen on Ru and Rh nanoparticles chemically modified with Se or S. Among the four chalcogen-modified specimens examined, the modification of Ru with Se shows the highest activity. [P. Zelenay, U.S. Pat. Appl. filed Dec. 5, 2005]. Therefore, the highlight of this study is the synthesis and use of the RuSe catalyst vs the cluster-type Rux Sey catalysts investigated before, and on providing evidence toward similar reactivity functions between RuSe and Rux Sey. On the nanoparticle RuSe electrode, ORR commences at 0.9 V and the diffusion-limiting reduction current is attained at ?0.4 V (vs a reference hydrogen electrode). ORR activity does not decrease in the presence of methanol, showing a full methanol tolerance at methanol concentrations investigated in this study. It is proposed that surface metallic Ru atoms embedded in Se matrices are the catalytic active sites to sustain ORR at a high level. However, the smooth Ru disk surface modified by selenium displays lower activity than a "clean" Ru disk (without Se) and a noticeably lower activity than the nanoparticle RuSe. Possible reasons for this behavior are discussed. Finally, a RuS catalyst is slightly more active than a H2 -reduced Ru black catalyst but is much less active than RuSe and Rux Sey, and is also methanol-tolerant at 0.1 M methanol concentrations. Both Se and S reduce ORR rates on Rh. © 2006 The Electrochemical Society. All rights reserved.