Deposition of Sn on Pt(lll) electrode gives rise to a sharp redox peak which is not due to the adsorption/desorption of Sn. The catalytic effect of this Sn-UPD on the oxidation of adsorbed CO is negligible. UPD of Sn on Pt(332) and Pt(755) on the other hand, has a remarkable catalytic effect; the onset of CO oxidation is shifted down to 0.25 V. Comparative experiments are presented for Bi deposition on these surfaces. Even when the bismuth coverage is high enough to block H-adsorption completely, CO is coadsorbed. Impedance measurements show that the rate of hydrogen adsorption is decreased by Sn UPD on Pt(lll).