In order to learn more about the nature of the active Au species for CO oxidation on TiO2 supported Au catalysts, we studied the influence of different gold oxidation states on the CO oxidation activity at 80 degrees Constructurally well defined Au/TiO2 (P25) catalysts. For that purpose, the metallic Au-0 nanoparticles were selectively removed from the catalyst by cyanide leaching, while dispersed ionic Aun+ species would remain on the catalyst. The catalysts were prepared by deposition-precipitation. The composition of the active Au phase before leaching was varied by either using a fresh catalyst or calcining it in an O-2/N-2 mixture, and by applying different conditioning procedures after leaching. The size of the gold particles and their chemical state were characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), the CO adsorption and oxidation behavior was evaluated by kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. In the latter measurements, we did not detect any influence of ionic Au3+ on the reaction. Metallic Au-0 nanoparticles, obtained mainly by conditioning in O-2 atmosphere at elevated temperatures (400 degrees C), but also during the CO oxidation reaction on a N-2 pre-treated or fresh catalyst, seem to be a requisite for a high CO oxidation activity of Au/TiO2 catalysts. (C) 2014 Elsevier B.V. All rights reserved.