Two Au/TiO2 samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au-0-CO at around 2107 cm(-1); (ii) Au+-CO at ca. 2175 cm(-1); (iii) Au delta+-CO in the region of 2140-2137 cm(-1) and (iv) Au delta'+-CO (delta(') > delta) at around 2155 cm(-1). The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Au delta+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge delta+ (0 < delta < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Au delta+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O-2 mixture. It resulted in creation of Au delta'+ sites with a higher positive charge than that of the Au delta+ sites. In this case the oxidation involved a higher number of Au-0 sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of "isolated" Au+ species. The results indicate that small metal particles are more easily oxidized by a NO + O-2 mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed.