The influence of the Nafion ionomer as a proton conductor on the platinum-ruthenium (Pt-Ru) alloy electrocatalyst for the electro-oxidation of methanol was investigated using combinatorial electrochemical methods. The most active composition of the Pt-Ru alloy electrocatalysts for the electro-oxidation of methanol was Pt(54.5)Ru(45.5), and the most active composition of the Pt-Ru-Nafion electrocatalysts for the electro-oxidation of methanol was metal (Pt:Ru = 1: 1, molar ratio):Nafion = 63.6:36.4, wt.%. The electrochemical properties of the electrodes were compared using cyclic voltammetry, impedance spectroscopy and a performance test of a direct methanol fuel cell (DMFC). When the Nation was added to the Pt-Ru alloy electrocatalyst, the initial activation potential decreased about 80 mV versus RHE, and the methanol electro-oxidation activity simultaneously increased. In a CO poisoning experiment, the CO was desorbed at a lower potential (lower than 130 mV versus RHE) when the Nafion was present. These results suggest a synergistic effect of Nafion-active metal catalysts for the electro-oxidation of methanol. (C) 2003 Published by Elsevier Science B.V.