Sb modified Pt(lll) electrodes have been characterized and their catalytic activity toward the oxidation of formic acid has been investigated. At low and intermediate coverages, irreversibly adsorbed Sb on Pt(lll) electrodes has a similar behavior to that of Bi adsorbed on Pt(lll). The voltammetric profile exhibits a peak at 0.49 V that corresponds to the surface oxidation of Sb to Sb(II) in form of an oxygenated species, each Sb adatom blocking three platinum sites. The Sb adlayer can be desorbed at potentials above 0.80 V without disruption of the long range order of the Pt(lll) surface. Regarding the catalytic activity of the low Sb coverages on Pt(lll) electrodes, these surfaces are able to inhibit poison formation from formic acid through a long range effect. For the direct oxidation of formic acid, Sb increases the catalytic current for formic acid oxidation at 0.5 V (the same potential value as on Pt(lll)) in a similar way as observed for Bi or As adsorbed on Pt(lll) electrodes. At high coverages, adsorbed Sb forms likely a surface alloy, probably by place exchange with the surface Pt atoms. The desorption of the Sb leads to the disruption of the long range order of the Pt(lll) surface, in a similar way (although at lower extent) to that obtained by successive cycles in the oxygen adsorption region. When the alloy is formed, a new oxidation wave for formic acid is observed at 0.2 V, indicating a high increase of the catalytic activity of the surface toward formic acid oxidation.