Significant enhancements in the electrocatalytic oxidation of HCOOH are obtained on deliberately stepped platinum surfaces with (111) terraces and (100) monatomic steps when bismuth is deposited on terrace and/or step sites. The bismuth-modified surfaces with higher step densities/narrower terrace widths exhibit higher catalytic activity in the oxidation of HCOOH than surfaces with lower step densities/wider terrace widths, behavior which is opposite to that exhibited by the analogous bare (unmodified) surfaces. For Bi/Pt(544) and Bi/Pt(755), with nine- and six-atom wide (111) terraces, respectively, maximum catalytic behavior occurs when bismuth is deposited on all step sites and partially deposited on the terraces. On the other hand, for Bi/Pt(211) and Bi/Pt(311), with three- and two-atom wide (111) terraces, respectively, maximum catalytic behavior occurs when bismuth is deposited only on the steps, suggesting that a critical ensemble (critical size), two atoms wide, enhances the reactive intermediate pathway for the complete oxidation of HCOOH over the poisoning reaction pathway. Moreover, the potential for the onset of oxidation of HCOOH shifts to less positive potentials as the terrace width of the bismuth-modified surfaces becomes narrower, suggesting a diminution in the activation energy barrier as the ensemble size becomes smaller. Contrary to the behavior on the stepped surfaces, bismuth-modified, polyoriented spherical platinum electrodes, which contain a random distribution of (111), (100), and (110) faces, exhibit a continuous rise in catalytic activity for HCOOH oxidation with bismuth coverage, even at bismuth coverages approaching 100%.