The adsorption of CO on planar Au/TiO2 model catalysts was studied by polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS) under catalytically relevant pressure (10-50 mbar) and temperature (30-120 degrees C) conditions, both in pure CO and in CO/O-2 reaction gas mixtures. The adsorption energy of CO on the Au particles was determined by a quantitative analysis of the temperature dependence of the CO absorption intensity in adsorption isobars. The data reveal considerable effects of the An particle size when pure CO is used; the initial adsorption energy decreases from 74 kJ mol(-1) (2 nm mean Au particle diameter) to 62 kJ mol(-1) (4 nm). For CO/O-2 gas mixtures, the initial CO adsorption energy is, irrespective of the Au particle size, constant at 63 kJ mol(-1) (i.e., the CO adsorption energy is reduced for smaller An particles), but this effect vanishes for larger Au particles. (C) 2007 Elsevier Inc. All rights reserved.