The electrocatalytic activity of commercial carbon supported PtRu/Vulcan and Pt3Sn/Vulcan bimetallic catalysts (E-TEK, Inc.) for ethanol oxidation under well defined electrolyte transport conditions and their selectivity for complete oxidation were evaluated using cyclic voltammetry combined with on-line differential electrochemistry mass spectrometry (DEMS) measurements and compared to the activity/selectivity of standard Pt/Vulcan catalysts. The main reaction products CO2, acetaldehyde and acetic acid were determined quantitatively, by appropriate calibration procedures, current efficiencies and product yields were calculated. Addition of Ru or Sn in binary Pt catalysts lowers the onset potential for ethanol electro-oxidation and leads to a subtle increase of the total activity of the Pt3Sn/Vulcan catalyst. It does not improve, however, the selectivity for complete oxidation to CO2, which is about 1 % for all three catalysts under present reaction conditions-incomplete ethanol oxidation to acetaldehyde and acetic acid prevails on all three catalysts. The results demonstrate that the performance of the respective catalysts is limited by their ability for C-C bond breaking rather than by their activity for the oxidation of poisoning adsorbed intermediates such as COad or CHx,ad species. (c) 2005 Elsevier B.V. All rights reserved.