This paper reports the effect of bismuth on the formation of CO from formic acid on platinum stepped surfaces containing six atoms wide (111) terraces and monoatomic steps having (110) or (100) symmetry. In both cases the steps were elaborated by irreversibly adsorbed bismuth in such a way that CO should be produced on the remaining terrace sites. It was found that bismuth decoration of (100) steps causes a large decrease in CO formation, whereas the effect of the same adatom when adsorbed on (110) steps is almost negligible. In this latter case it is necessary to adsorb bismuth on the terrace sites in order to obtain a similar inhibition of the poisoning reaction.