Current efficiencies for the formation of CO2 during methanol oxidation at smooth polycrystalline platinum electrodes were determined by differential electrochemical mass spectrometry in a thin layer flow through cell. In all cases, the current efficiencies are below 60%; in particular, values as low as 16% were found in 0.1 M methanol solution at 0.6 V, which shows that a large amount of soluble intermediates (formaldehyde and/or formic acid) are formed. The extent to which these soluble intermediates are further oxidized to CO2 depends on the diffusion conditions. For methanol oxidation a parallel oxidation path via COad is also active. The influence of the surface crystal structure and, in particular, of steps was also studied. Step decoration by foreign metals allowed examination of the effect of cocatalytic metals on well defined model surfaces.