The influence of oxygen (O 2 ) and carbon monoxide (CO) on Au nanoparticles supported on TiO 2 (110) in the size range of 2–3nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300K for O 2 and/or CO pressures of 0.1–1mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14–0.2 ML, particle size estimated to ∼2–2.5nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3nm). At elevated O 2 pressures, a new Au 4f component at higher binding energy (2.4–2.6eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2–2.5nm particles. The relative intensity of the oxide component depended strongly on O 2 pressure and, thus, on the equilibrium coverage of O 2 . While not present in 0.1mbar O 2 regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2–2.5nm in size at 1mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O 2 . The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles.