The maximum current density for HCOOH oxidation on deliberately stepped platinum surfaces with Pt(111) terraces of varying width and (110) monatomic steps decreases as the step density increases, or as the terrace width decreases. The maximum current densities on these surfaces are 1.6, 0.58, 0.46, and 0.39 mA cm(-2) for Pt(111), Pt(554) (nine-atom wide (111) terrace), Pt(332) (five-atom wide terrace), and Pt(221) (three-atom wide terrace), respectively. When the step sites are modified by irreversibly adsorbed bismuth adatoms, the trend is reversed, and the current densities for HCOOH oxidation are enhanced to 5.4, 9.9, and 17.3 mA cm(-2) for the Pt(554), Pt(332), and Pt(221) surfaces, respectively. The trend continues when bismuth is deposited on the terraces, where the maximum current densities normalized to the (111) terrace width for HCOOH oxidation are 2.8, 4.3, and 8.5 mA cm(-2) per terrace atom for the Pt(554), Pt(332), and Pt(221) surfaces, respectively. That the oxidation of HCOOH on these stepped Pt surfaces is enhanced on narrow (111) terraces with high (110) step-site densities modified with bismuth suggests that a decrease in the reaction ensemble size increases its rate. In addition, bismuth appears to block the formation of strongly adsorbing reaction intermediates/poisons on the reactive step sites, and may also contribute via electronic effects to the oxidation of HCOOH.