Potentiostatic investigations of the methanol oxidation in the absence and presence of oxygen at the three-phase boundary of a Pt-catalyzed gas diffusion electrode were carried out at several electrode potentials. In the presence of oxygen, a purely chemical reaction between oxygen and methanol was observed, with the products being CO2 and water. A study of the rate of CO2 formation and of the electrochemical currents for methanol oxidation showed that the rate of fuel consumption at the electrode is strongly increased by this chemical pathway. This is accompanied by a considerable decrease in the electrochemical currents for oxygen reduction. In the absence of oxygen and for potentials above 1.1 V vs. RHE, current oscillations were observed during the anodic process, which were attributed to the formation of an unstable layer of surface platinum oxides.