The electrochemical oxidation of methanol and formic acid in 1 M HClO4 is studied on boron-doped diamond electrode using differential electrochemical mass spectrometry (DEMS). DEMS is used to identify products and intermediates of the reaction monitored online during the potential sweep. The measurements show that the oxidation of both methanol and formic acid proceeds close to the potential of the oxygen evolution reaction, resulting in the appearance of an oxidation wave. During the oxidation, the oxygen evolution reaction shifts toward a higher potential, i.e., becomes a secondary reaction. Depending on the potential, either the formation of intermediates (partial oxidation) or the complete mineralization of organics, simultaneous with oxygen evolution, is observed. Diffusion limitation of the oxidation is clearly observed in parallel to the increase in O-2 formation. Methylformate is formed from methanol at potentials at which the local concentration of OH radicals is not sufficient for a complete mineralization. Formic acid oxidation proceeds in two waves; before the onset of oxygen evolution, an intermediate (probably oxalic acid) is produced in parallel to CO2 formation. Only in parallel to oxygen evolution is mineralization to CO2 is complete. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3207009] All rights reserved.