The influence of stabilized, underpotential deposited (upd) lead on the oxidation of formic acid and its strongly adsorbed species on a platinum electrode in has been investigated by measuring simultaneously the rate of production from both bulk and adsorbed formic acid. The use of on‐line mass spectrometry (MS) technique (DEMS) allows a discrimination of the products between adsorbed and bulk species with the help of isotope‐labeled 13C formic acid as a probe. The pronounced catalytic effect of upd lead adatoms on the bulk formic acid oxidation strongly depends on the coverage degree of adatoms. In addition, upd Pb facilitates the oxidation of formic acid adsorbates. However, the oxidation of formic acid cannot take place on a Pt surface covered up to 100% with upd lead adatoms and adsorbates. The results prove once more that a third‐body effect cannot account for the observed catalytic effects. An electronic interaction between the upd Pb and the Pt substrate is more likely. This results in an increase in rate of electrosorption on the modified free‐platinum surface.