Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltarnmetry has been employed to investigate the electrooxidation of formaldehyde, methyl formate, and dimethoxymethane (byproducts in methanol oxidation) on platinum in acidic solution. Owing to the sensitivity of SEIRAS being much higher than that of infrared reflection-absorption spectroscopy (IR-RAS), the reactions could be studied during cyclic voltammetric measurements at relatively fast potential sweep rates (50 mV s(-1) or faster). In the oxidation of formaldehyde, a weak band assigned to the symmetric O-C-O stretching mode of formate adsorbed on the Pt surface in a bridging configuration was observed for the first time in addition to those for linearly and bridge-bonded CO known to be poisons. The oxidation current was found to increase as the band intensity of formate increases. On the other hand, none of the other reactive intermediates claimed in earlier IR-RAS studies was detected. These results led us to propose that adsorbed formate is a reactive intermediate en route to CO2 production and the oxidation of formate to CO2 is the rate-determining step. Methyl formate and dimethoxymethane are also oxidized to CO2 via formate. (C) 2003 Published by Elsevier B.V.