In this work we investigate the electro-oxidation of CO and glycerol in acidic media on Pt (100) preferentially oriented nanoparticles unmodified and in presence of Sn adatoms with several coverage degrees. The voltammetric characterization in HClO4 shows that Sn adsorption affects the whole surface, but each domain presents a specific response. Namely, the adsorption of Sn follows the sequence: (100) terraces > (100) short domains > (110) domains. By using CO as surface probe, we demonstrate that the influence of Sn may be both beneficial and detrimental, depending on the nature of the site in which it adsorbs. Specifically, the electro-oxidation of CO on short (110) and (100) domains is anticipated in the presence of Sn, whereas it is hindered on (100) terraces. Concerning the electro-oxidation of glycerol, the influence of Sn is also complex, because Sn adatoms leads to a sensible change in the ways glycerol adsorbs on the surface. Such change anticipates the oxidation steps through a pure bifunctional mechanism that takes place along the surface defects and one-dimensional domains. On the other hand, in bidimensional (100) domains Sn causes a third-body effect that restrains the rupture of C-C bonds. Consequently, in situ FTIR analysis shows that the remaining free surface favors the production of species containing carbonyl groups. The global output suggests that the proper combination of well-designed electrocatalysts and adatoms allows the control of the pathways involved in the oxidation of small organic molecules. (C) 2018 Elsevier Ltd. All rights reserved.