The influence of a stabilized submonolayer of upd tin adatoms on the oxidation of adsorbed and bulk formic acid on sputtered platinum electrodes in 0.1 M HClO4 has been investigated by measuring simultaneously the rate of CO2 production from both adsorbed and bulk formic acid. The use of on-line MS technique (DEMS) enables us to distinguish the volatile products (CO2) formed in the oxidation of adsorbed and bulk species, respectively, with the help of isotope-labeled C-13-formic acid as a probe. A considerable promoting effect of upd tin adatoms on the oxidation of the bulk formic acid was observed. This catalytic activity depends strongly on the coverage of the tin. On the other hand, the oxidation of the formic acid adsorbate is accelerated significantly in presence of Sn adatoms on the surface or tin ions in solution. Based on our results, a discrimination between the electrocatalytic models of tin adatoms postulated in literature was given. We suggested that a combination of a bifunctional mechanism and an electronic interaction between the upd Sn adatoms and Pt substrate could account for the pronounced catalytic influence on the oxidation of formic acid.