The coverage of Sn on Pt(111) which is obtained by electrochemical deposition from 5 x 10(-5) M Sn2+ in 0.5 M H2SO4 has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of v(max) = 0.35 (Sn to surface Pt atoms).Co-adsorption of CO with Sn on Pt(111) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(111)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm(-1) band associated with bridge-bonded CO and the 2070 cm(-1) band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm(-1) band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2 x 2)-3CO saturation structure on Pt(I 11). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(I 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV (C) 2003 Elsevier Ltd. All rights reserved.