Pt(111) possesses lower catalytic activity for the HCOOH oxidation in acidic solution than do Pt(100) and Pt(110), but in the presence of Pb adatoms, it shows strong catalytic action. Such a catalytic effect of Pb adatoms on Pt(111) has been investigated using cyclic voltammetry (cv), differential electrochemical mass spectrometry (DEMS) and the modulated potential-time waveform (mptw) technique, also with the "solution substitution" experiments. It was found that : (1) the adsorbed Pb(0)/Pb(II) redox couple remains on the surface during at least several potential cycles in 0.1 M HClO4 and Pb(0) shows strong catalytic action; (2) less than one third of Pb monolayer is enough to develop the catalytic action and retards the formation of poisonous CO; (3) an abrupt change in the oxidation current occurs corresponding to the change in the Pb oxidation state at the ’transient region’ potential; and (4) the;catalytic effect cannot easily be explained by a simple model, such as the "third-body" effect or "bifunctional theory". The cooperation of the molecular orientation on the surface with the surface state as modified by Pb adatoms should be taken into account.