The oxidation of formaldehyde in perchloric acid solutions has been investigated by cyclic voltammetry and electromodulated infrared reflectance spectroscopy (EMIRS). The results show that the hydration-dehydration equilibrium of formaldehyde plays an important role in the formation of the strongly bonded layer. The CO species are detected by EMIR spectra in the linearly bonded, bridge-bonded and multibonded configurations. It is shown that the methanol present in formaldehyde solutions as stabilizer increases the rate of CO formation. CO is adsorbed first in the bridge-bonded form, and then high coverages favour the linear structure. An aldehydic carbonyl band is found which must be involved in the formation of the CO layer. Bands corresponding to carbonyl acid groups are found and are attributed to reactive intermediates.