We have investigated the underpotential deposition (UPD) of silver on high-index platinum single crystals vicinal to the (1 1 1) plane. Carbon monoxide displacement experiments were carried out on Ag-modified Pt(6 6 5) and Pt(3 3 2) stepped single crystal electrodes and examined by means of electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry (CV) in 0.05 M sulfuric acid. Analogous to studies of Pt(1 1 1) by Klein [L.H. Klein, Etude des etats d'adsorption electrochimique sur les electrodes de platine et de rhodium monocristallins modifiees ou non par les adatomes, Ecole Nationale de Chimie de Paris, Universite Paris 6, Paris, 1997] and Domke et al. [K.F. Domke. X.-Y. Xiao, H. Baltruschat. PCCP 10 (2008) 1555], carbon monoxide displaces Ag from stepped Pt electrodes. The present STM studies reveal the formation of small, biatomic thick or thicker Ag clusters on Pt(66 5) and Pt(3 3 2) after co-adsorption of CO, extending across step edges, They have a narrow size distribution centred around 10 nm diameter independent of the substrate step density. The desorption of the second Ag adlayer and of CO are monitored electrochemically in two well-separated oxidation peaks at around 0.68 and 0.9 V vs RHE, respectively, whereas the desorption peak of Ag of the first adlayer cannot be completely separated from the early onset of Pt oxidation at approximately 1.13 V vs RHE. (C) 2009 Elsevier Ltd. All rights reserved.