Electrochemical reduction of CO2 at a platinum electrode was studied in 0.1 M tetraethylammonium perchlorate acetonitrile-water mixtures at constant current density of 5 mA/cm(2). Pt, inert in the electrochemical reduction of CO2 in aqueous media, can reduce CO2 mainly to oxalic acid in water free acetonitrile. With the increase of water concentration, oxalic acid formation dropped and the yield of formic acid increased. CO2 reduction was suppressed and hydrogen evolution prevails with further increase of water concentration. The presence of adsorbed CO was confirmed on the PI electrode surface by in situ Fourier transform-infrared reflection absorption spectroscopy in the electrolyte. The molecular ratio CO2/H2O in the electrolyte solution apparently determines whether or not CO2 is reduced at the CO covered Pt electrode. The product selectivity was compared with the results obtained from Pb and. Au electrodes. The CO covered Pt electrode behaves like the Pb electrode, giving oxalic acid as the major product in acetonitrile with low water concentration, whereas the Au electrode yields CO. The electrocatalytic property of CO covered Pt in CO2 reduction is discussed in connection with the adsorbability of CO2.- formed in the initial step of CO2 reduction as previously proposed by the present authors. The solubility of CO2 in acetonitrile-water mixtures, measured by neutralization titration, is also presented.