Ambient pressure photoelectron spectroscopy (APPES) has been used to study the adsorption of nitric oxide (NO) and molecular oxygen (O-2) over gold-based model catalysts consisting of mono-dispersed gold nanoparticles with different diameters (2-5 nm) and oxide supports (including polycrystalline silica and titania thin films). APPES is an in situ technique that makes possible to monitor via XPS chemical changes occurring on the catalyst surface and to identify adsorbed species under reaction conditions. In our experiments, no changes were observed on the Au/SiO2 samples during exposure to 0.5 Torr of NO, while adsorbed NO and several N-containing species were detected on Au/TiO2 model catalysts under the same conditions. In addition, shifts in the Ti3p and O 1s peaks in TiO2 were observed relative to the Au 4f peak. Similar behavior, although to a lesser extent, was observed on Au/TiO2 samples when O-2 was used. In both cases, the shifts of the Ti3p and O 1s peaks could be attributed to band bending effects on the TiO2 substrate caused by chemisorption of the gases. (C) 2011 Elsevier Inc. All rights reserved.