The formation of formaldehyde during methanol electrochemical oxidation on supported Pt and Pt-Ru catalysts was investigated. While on solid platinum electrodes, the formaldehyde yields from methanol oxidation are near 30% at low potentials; the yields fall below 2% for methanol electrochemical oxidation on carbon-supported catalysts in Nafion. The lower formaldehyde yields, which result from more complete methanol oxidation, are believed to arise from the ability of partial oxidation products to be transported to an array of active catalyst sites dispersed within the three-dimensional network of the Nafion film.