The effect of particle size of ultrafine palladium particles on the electrochemical oxidation of formic acid in a perchloric acid solution has been investigated by cyclic voltammetry (CV) and differential electrochemical mass spectroscopy (DEMS). Except in the hydrogen sorption-desorption region, only carbon dioxide was observed by the DEMS as an oxidation product, and the profile of the i(mass)-E curve was in fair agreement with that of i-E curves. With a decrease in the palladium particle size, the specific activity of the palladium particles for the oxidation of formic acid gradually increased until ca. 3 nm in the mean diameter, and then it steeply decreased.