Formaldehyde formed in the electrochemical oxidation of methanol was measured in order to assess its importance as a reaction intermediate and potential source of efficiency loss in direct methanol fuel cells. Formaldehyde generated from 15 mM methanol in 0.1 M HClO4 at fixed potentials with a small volume electrolysis arrangement was determined with a sensitive fluorescence assay. The formaldehyde yields approached 30% of the total electrolysis charge at 0.2-0.3 V (vs Ag/AgCl). The percentages dropped at more positive potentials, as other oxidation pathways became dominant. However, formaldehyde production continued to increase with potential, maximizing near 0.5 V. This study demonstrates that formaldehyde, which is often not detectable with modern in situ analysis techniques, can be produced during methanol electrochemical oxidation in significant amounts. A fluorescence assay specific for formaldehyde is suggested for use in parallel with in situ measurements.