The favourable influence of selenium on the catalytic properties of Ru-based catalysts for the oxygen reduction reaction in acid electrolytes has been investigated by rotating disk electrode measurements. Compared to the oxygen reduction of selenium-free Ru-based catalysts, the overpotential at low current densities (ca. 10 muA. cm(-2)) is not affected by the presence of selenium whereas selenium-containing catalysts show higher current densities under fuel cell relevant conditions. The kinetically controlled current density at 0.6 V versus SHE increases 4-5 fold with increasing selenium content. A maximum value is obtained at about 15 mol% Se. This effect is tentatively explained by a modification of the catalytic active centre, which is assumed to consist of Ru-C-CO complexes. IR spectroscopic investigations indicate a reaction of selenium with these complexes. This model is also supported by the study of the electrooxidation of CO. In contrast to the selenium-free catalyst, no CO oxidation is observed on the selenium-containing catalyst. Additional effects of selenium are an enhanced stability towards electrochemical oxidation and a lower amount of Ru oxides formed during synthesis, as evidenced from XRD investigations. Direct four electron oxygen reduction to water is efficient and H2O2 production of these catalysts is small (about 5% at potentials < 0.3 V vs. SHE).