In this work we study the kinetics of the oxygen reduction reaction on carbon-supported Ru nanoparticles modified with various amounts of Se. Rotating disk electrode is used to determine kinetic currents for the ORR in 0.1 M H2SO4 at 298 K and O-2 partial pressures from 1 to0.01 atm. The dependence of the ORR activity on Se/Ru ratio shows volcano-type behavior with ca. 10 fold increase of the mass activity at 0.1 < Se/Ru < 0.3. The reaction order in O-2 is close to I in the interval of overpotentials from 0.4 to 0.7 V, and is independent of the presence of Se. Regardless the amount of Se, the Tafel slope demonstrates continuous increase from ca. 70 mV/dec at 0.4 V to ca. 140 mV/dec at 0.6 V overpotential. In situ EXAFS spectra are measured at Ru K-edge (in the transmission mode) and Se K-edge (in the fluorescence mode) in argon and oxygen saturated 0.1 M H2SO4 solutions in the interval of electrode potentials from 0.050 to 0.750 V RHE. The data are used to explore the surface state changes of Ru and RuxSey particles and clarify the promoting role of Se during the ORR. (C) 2009 Elsevier B.V. All rights reserved.