We have found that the formic acid oxidation on Pt does not produce current in the potential range between 0.05 and 1.4 V (vs. a hydrogen electrode) in the absence of water, while the methanol oxidation produces a large current in the absence of water. Investigations by surface-enhanced infrared absorption spectroscopy (SEIRAS) simultaneously with electrochemical measurement clarify that, for formic acid oxidation, adsorbed CO is not oxidized even at 1.4 V due to the absence of water, and thus practically no current is produced. On the other hand, for methanol oxidation, the amount of adsorbed CO decreases at a high potential, which leads us to think that adsorbed CO reacts with methanol to form methyl formate to produce vacant sites, where methanol is oxidized to form formaldehyde or methyl formate. With increasing water concentration for formic acid oxidation, the current in the negative-going potential sweep grows larger due to the increasing number of vacant sites produced by the oxidation of adsorbed CO with water, hydroxide or oxide, the last two are formed from water. Potential oscillations during oxidation of formic acid and methanol with no or low water concentrations have also been investigated. (C) 2014 Elsevier Ltd. All rights reserved.