The changes in the optical properties of Au/TiO2 powder catalyst, prepared by the deposition-precipitation method, are measured in the visible range by use of a home-made diffuse differential reflectance spectrometer, when the ambient atmosphere is switched between H2 and O2. Two main features are observed: (i) a short wavelength one, located between 400 and 600 nm, is shown to be related to the modifications of the plasmon resonance of the 3-dimensional metallic gold particles with mean size around 4 nm, induced by the exposure to oxygen or by its removal; (ii) a second feature, observed at long wavelengths between 600 and 1100 nm, is extremely sensitive to the exposure to oxygen. This optical feature could be due to the presence in the catalyst, either of Au0 clusters of several tens atoms, which are expected to display molecular-like transitions in this optical range, or to specific sites of the Au particles (edge atoms or peripheral atoms at the interface with the TiO2 support), which could be highly reactive to oxygen.