The adsorption and oxidation reaction of CO on gamma-Al2O3 single-crystal thin films with Au nanoparticles were studied as a function of the particle size using reflection-adsorption infrared spectroscopy, temperature-programmed desorption (TPD), and the molecular beam technique in a temperature range of 93 K < T-s < 213 K. It was found that CO undergoes a precursor adsorption on the Au particles ending up in an on-top position. The activation energy of this process is calculated from the molecular beam data to (E-d - E-a) = 3.53 kJ/mol. The amount of CO adsorbed increases with increasing particle size. Two types of adsorption sites could be deduced from the TPD data. The oxidation of CO, however, was never observed in the present experiments.