Methanol oxidation has been investigated at a sputtered Pt film electrode by using in situ Fourier transform infrared spectroscopy with the attenuated total reflection technique, which can identify directly adsorbed species on the electrode surface. Linear CO, bridged CO, formyl species, and formic acid-related species have been clearly identified during the electro-oxidation of methanol in the intact cyclic voltammogram between 0.05 and 1.0 V, where the initial potential is applied at 0.05 or 1.0 V. The formyl species, linear CO, and bridged CO adsorb on the Pt surface at a low potential, for example, 0.05 V. A potential-induced conversion between the linear CO, the bridged CO, and the formic acid-related species is observed during the methanol oxidation. It was clear that an oxidation via formic acid, at least up to formic acid, is the predominant route for the methanol oxidation in a potential range from 0.2 V to ca. 0.6 V and the oxidation is prohibited by the high coverage of Pt sites with CO. In a higher potential region than that, the CO and HCOOH parallel routes are opened for the complete methanol oxidation. The correlation between the methanol oxidation and the oxides formed on Pt surface is also discussed, which affects the reactivity of the electrode and the reaction mechanism.