Current efficiencies of CO2 during methanol oxidation were determined by differential electrochemical mass spectrometry (DEMS). The influences of methanol concentration, electrode structure and ruthenium on methanol oxidation have been investigated. CO2 is formed via COad, oxidation in a serial reaction, while dissolved intermediates, i.e. formaldehyde and formic acid are also formed in a parallel reaction. The chance that these intermediates can further be oxidised depends on the diffusion and convection conditions in connection with the surface structure of the Pt electrode. High methanol concentrations favour the reaction path via dissolved intermediates. Ru promotes methanol oxidation to CO2 via COad, leading to a higher current efficiency of CO2.