Methanol oxidation on a supported Pt fuel cell catalyst was investigated by on-line differential electrochemical mass spectrometry (DEMS) at continuous electrolyte flow and defined catalyst utilization, employing a thin-film electrode setup and a thin-layer flow-through cell. The active surface of the Pt/Vulcan (E-TEK) high surface area catalyst was characterized quantitatively by H-upd and preadsorbed CO monolayer stripping. Methanol stripping DEMS experiments, oxidizing the adsorbed dehydrogenation products formed upon methanol adsorption at potentials in the hydrogen adsorption region, show that the coverage of these products and hence the methanol uptake depend on the electrode potential, in contrast to the potential-independent COad coverage. The dehydrogenation products cannot be displaced by H-upd. The number of close to two electrons used per oxidation of one adsorbed dehydrogenation product identifies this as COad species. Further methanol dehydrogenation is hindered when the CO adlayer reaches a density of 1/3 monolayers. Side reactions during bulk methanol oxidation were identified directly by DEMS, showing methylformate formation in addition to the main product, CO2. The extent of formaldehyde and formic acid formation was estimated from mass spectrometric and faradaic currents to be between 25% and 50% per dehydrogenation step. The exclusive formation of fully deuterated methylformate upon oxidation of deuterated methanol underlines the irreversibility of methanol dehydrogenation and rules out H/D exchange. A rather low kinetic H/D isotope effect implies that the removal of poisoning COad intermediates rather than C-H bond dissociation determines the methanol oxidation rate, although there is a contribution from the latter step. Reduction of an anodically preformed PtO monolayer by methanol under open-circuit conditions indicates that Pt oxy species are equally active for methanol oxidation.