We report a new method of preparation of a methanol-tolerant Se/Ru cathode catalyst for the direct methanol fuel cell (DMFC) [1,2], whereby selenium is deposited on ruthenium nanoparticles by H(2)-reduction of SeO(2) in aqueous solution at room temperature. The obtained Se/Ru(aq) was studied by electrochemical measurements and tested as a cathode catalyst in H(2)-air and direct methanol fuel cells. The new catalyst formulation (Se/Ru(aq)) is shown to be superior to Se/Ru synthesized from xylenes solvent [3] and to Ru black by RRDE measurements, in terms of both activity and selectivity for complete oxygen reduction to water. Although Ru black is less active, the Tafel slopes and activation energies of Se/Ru catalysts and reduced-Ru black are similar, implying similar ORR mechanisms. In H(2)-air fuel cell tests, Se/Ru(aq) was more active than Se/Ru(xyl) at all voltages. Compared to Ru black, Se/Ru(aq) was superior at low current densities, but Ru black slightly exceeded the performance of Se/Ru at high current densities. To explain the RRDE and fuel cell observations, the two roles of Se as an inhibitor of Ru oxidation and as a site-blocker are discussed. (C) 2011 Elsevier B.V. All rights reserved.