Electro-oxidation of ethanol on a PtRu/C catalyst at intermediate temperature (250 degrees C) was investigated by electro-chemical measurement and analysis of reaction products using cesium dihydrogen phosphate as a proton-conducting electrolyte. High CO2 selectivity in the ethanol electro-oxidation was observed, which suggests that total oxidation of ethanol proceeds effectively at the intermediate temperature. Formation of CH4, a major byproduct in ethanol electro-oxidation on Pt/C catalyst at intermediate temperature, was suppressed by Ru addition, while CH3CHO formation was accelerated by Ru addition. Detailed kinetic analysis of the electrode reaction mechanism revealed that Ru addition accelerates ethanol dehydrogenation and water dissociation in ethanol electro-oxidation. Furthermore, OH formed in the fast water dissociation enhances oxidation of surface C-1 adsorbates, which are produced in fast C-C bond dissociation of ethanol on Pt at intermediate temperature. These results suggest the effectiveness of Ru addition in aiming for the total oxidation of ethanol at intermediate temperature.