WO3-modified carbon-supported bi-component ruthenium-selenium, RuSex (Ru, 20; Se, 1 wt%), nanoparticles were dispersed in the form of Nafion-containing inks on glassy carbon electrodes to produce electrocatalytic interfaces reactive towards electroreduction of dioxygen in acid medium (0.5 mol dm(-3) H2SO4). It was apparent from the rotating disk voltammetric experiments that the reduction of oxygen proceeded at WO3-modified electrocatalyst at more than 100 mV more positive potential when compared to bare (WO3-free) RuSex system (that had been prepared under analogous conditions and deposited at the same loading of 156 mu g cm(-2)). The ring-disk rotating voltammetric measurements show that, while the production of hydrogen peroxide intermediate was significantly lower, the kinetic parameter (heterogeneous rate constant) for the oxygen reduction was higher for WO3-modified RuSex (relative to bare RuSex). Comparison was also made to highly-efficient Vulcan-supported Pt or Pt/Co nanoparticles: while the half-wave potential for the oxygen reduction at WO3-modified carbon-supported RuSex was still more negative relative to the potentials characteristic of Pt-based electrocatalysts, the oxygen reduction rotating disk voltammetric current densities (measured at 1600 rpm) were almost identical.