The inhibition of O-2 reduction On a Pt(lll) electrode surface with Ag adatoms has been studied to gain information on the type of O-2 adsorption during the reaction. A pronounced inhibition is observed upon adsorption of Ag submonolayers, and a complete inhibition is caused by underpotential deposition of a pseudomorphic Ag monolayer and of an incommensurate hexagonal bilayer. Electrochemical and in situ surface X-ray scattering techniques were used to obtain kinetic and structural data. Statistical models for the inhibition of O-2 reduction caused at submonolayers of Ag adatoms residing on top, bridge, and hollow sites of Pt(lll) for O-2 adsorbed on the same type of sites in three different configurations have been compared with the experimental data. Analysis of the extent of the inhibition of O-2 reduction as a function of the Ag coverage shows that the data are best interpreted with O-2 adsorbed at a bridge site.