The mutual effects of the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) occurring simultaneously on a Pt/Vulcan fuel cell catalyst in O-2-saturated 0.5 M H2SO4 Solution containing 0.1 or 0.01 M methanol and the formation of a mixed potential were studied using differential electrochemical mass spectrometry (DEMS). This allows to separately monitor the partial reaction rates via CO2 formation (MOR) and O-2 consumption (ORR) and determine the partial Faradaic currents for the individual reactions. While the influence of methanol and of the MOR lead to significant changes in the ORR characteristics, e.g., a shift of the cathode potential by about 0.3 V in 0.1 methanol, the mutual effects on the reaction characteristics of the two partial reactions are much smaller. A fit of the sum of the partial currents to the measured net current results in electrons yield of 8.3 electrons (0.1 M methanol solution) and 6.3 electrons (0.01 M methanol solution) per CO2 molecule for the MOR, compared to values of 7.5 and six electrons in the absence of O-2, indicating an O-2-induced increase in incomplete methanol oxidation by-products. Slight deviations from a simple additive superposition of the ORR and MOR currents, the 'classic' picture for mixed potential formation, are mainly caused by O-2-induced changes in the MOR pathways, together with small changes in the O-2 transport, while the ORR selectivity, predominant oxygen reduction to water, is maintained at relevant potentials. (C) 2004 Elsevier Ltd. All rights reserved.