The electrooxidation of formaldehyde on Pt(100) and Pt(110) electrodes in a perchloric acid medium was investigated by cyclic voltammetry and Electromodulated Infra-Red Reflectance Spectroscopy (EMIRS). Both electrodes present two oxidation processes occurring during the positive potential scan. These processes involve either direct methylene-glycol oxidation or the formation of adsorbed CO species. The Pt(100) electrode is more active for the methylene glycol oxidation, which can be seen at low formaldehyde concentrations. The CO species were adsorbed in the linear form on both electrode surfaces, whereas multibonded and bridge bonded CO forms were only present on the Pt(100) electrode. The methylene glycol oxidation process is inhibited by the presence of adsorbed CO species which block the surface, Pt(110) being more blocked than Pt(100).