We studied the influence of preadsorbed sulfur on the adsorption of CO on Pt(355) and Pt(322) by in situ X-ray photoelectron spectroscopy. At low temperatures, no adsorption of CO at steps is found, indicating a passivation by sulfur. Upon heating, population of step sites with CO occurs. This behavior is attributed to a kinetic passivation of the steps by sulfur at low temperatures, which is lifted upon heating. The characteristic transition temperatures of 185 K for Pt(355) and 240 K for Pt(322) are significantly different, due to a higher activation barrier for the CO/S site exchange process for Pt(322). (c) 2007 Elsevier B.V. All rights reserved.