The effect of the particle size of ultrafine palladium particles supported on a flat and dense glassy carbon on the electrochemical oxidation of formaldehyde in a perchloric acid solution has been investigated by cyclic voltammetry (CV) and differential electrochemical mass spectroscopy (DEMS). A maximum for the specific activity of the oxidation of HCHO on palladium particles was observed at ca 4.3 nm in diameter. The current density for HCHO oxidation became very low with a decrease in the palladium particle size. As the oxidation product of HCHO, only carbon dioxide was detected by the DEMS as with the oxidation of HCOOH.