The amount of poison formed and oxidatively desorbed upon the dissociative adsorption of formic acid on bismuth-modified Pt[n(111) x 100 and 110] electrodes has been investigated and found to decrease linearly with bismuth coverage on the step sites. For Pt(544) and Pt(554) (i.e., nine atom wide (111) terraces with (100) and (110) steps, respectively), the amount of charge to oxidatively desorb the poison decreases from 80 and 220 mu C/cm(2), respectively, to similar values of 25 and 30 mu C/cm(2), respectively, when the step sites are completely blocked with bismuth and the (111) terraces are bare. As the step site density of the platinum surface is increased, the amount of charge corresponding to the adsorbed poison, when the step sites are completely blocked with bismuth, decreases on the surfaces with (100) steps but remains constant or slightly increases on the surfaces with (110) steps. As bismuth is deposited on the (111) terraces, the amount of poison formed on both electrodes is greatly diminished, so that when bismuth coverages of ca. 0.09 are attained on the (111) terraces, little poison is formed. While both the (100) and (110) stepped surfaces exhibit this rapid diminution in poison formation with bismuth coverage on the (111) terraces, the (100) stepped surfaces reach the minimum at a faster rate, suggesting that the (110) steps influence the surface by making it more reactive to the poison formation reaction, even when they are blocked with bismuth. For formic acid oxidation, the (110) stepped Pt surfaces are less active than the (100) stepped surfaces when bismuth is only adsorbed on the step sites but become more active when bismuth is adsorbed on both the steps and partially on the (111) terraces.