Ethanol electrooxidation reaction (EOR) pathways on polycrystalline platinum were studied with broadband sum-frequency generation (BB-SFG) spectroscopy and electrochemistry in unprecedented detail and under working fuel cell conditions. We present the first observation of adsorbed acetate and co-adsorbed sulfuric acid anions with SFG and a discussion of their relation to the EOR. Surface-adsorbed intermediates such as CO on Pt atop sites and acetate are observed in both H(2)SO(4) and HClO(4) solutions. However, CO molecules on bridge sites and sulfuric acid anions are found in H(2)SO(4) only. At E < 0.5 V vs. Ag/AgCl, CO is the predominantly adsorbed species. Increasing the potential to E > 0.5 V results in the oxidative removal of CO and the adsorption of acetate anions. Experiments with isotopically labeled ethanol ((12)CH(3)(13)CH(2)OH) reveal information on the carbon-carbon bond cleavage and the subsequent CO formation. In particular, the methyl fragment (-(12)CH(x)) produces far less (12)CO and suggests methyl electroreduction to methane and/or the persistence of -CH(x) on the Pt surface. (C) 2010 Elsevier Inc. All rights reserved.