The nature of the adsorption sites of CO and NO as they compete to adsorb on a Rh(111) surface has been determined using by X-ray photoelectron spectroscopy in situ, under gas mixtures of the reactants in the Pascal pressure range. It was found that near room temperature NO readily displaced CO from threefold hollow sites when the NO partial pressure was below 30%. CO is displaced from top sites only at higher NO pressures and after heating. The kinetics of CO:NO exchange at top sites is very slow at room temperature (hours), and requires heating for complete exchange. (C) 2004 Elsevier Inc. All rights reserved.