Poison formation reaction from formic acid has been studied on different platinum stepped surfaces modified by irreversibly adsorbed arsenic, antimony, bismuth, selenium and tellurium. These surfaces combine (111) symmetry terraces of different width and monoatomic steps having (110) and (100) symmetry. The employed adatoms can be classified in two categories: the adatoms that deposit preferentially on the step and can decorate it and those that do not decorate the step. The adatoms that are able to decorate the step show two different behaviors according to the terrace width for this poisoning reaction. Surfaces with narrower terraces exhibit a linear decrease in the total poison accumulation with the adatom coverage, whereas, for the surfaces with wide terraces, the total amount of poison diminishes sharply after all the step sites have been covered by the adatom. On the other hand, surfaces with adatoms that are not able to decorate the step always exhibit the linear behavior. All the results have been interpreted in terms of the electronic properties of the adatom and the surface. The main conclusion is that in surfaces with wide terraces, poison formation from formic acid only takes place on the step sites. This result can be extrapolated to the Pt(111) electrode, explaining the behavior of the poison formation reaction found for this electrode.