The influence of monatomic steps on the oxidation of adsorbed CO is examined using potentiodynamic and galvanostatic experiments. Galvanostatic experiments allow a better determination of the lowest possible oxidation potential than cyclic voltammetry. By comparing results on Pt(111), Pt(665), Pt(332), and Pt(755), we found that steps with a local (110) geometry are more active than those with a (100) geometry. Steps can be decorated by Ru, thus leading to surfaces with a known atomic arrangement of the components. During galvanostatic oxidation of adsorbed CO on such surfaces a second potential plateau is observed, the value of which depends on the density of steps. These transients suggest the existence of an electronic effect in addition to the widely accepted bifunctional mechanism: Due to the electronic effect, only CO in the neighborhood of Ru is destabilized, whereas CO adsorbed at more distant sites is not influenced and has to diffuse "uphill" to the reactive sites, leading to an apparently slow diffusion. In addition, the spillover effect according to the bifunctional mechanism lowers the height of the activation barrier for CO oxidation and therefore has an influence on all adsorbed CO.