A Ru/TiO2 (anatase) sample prepared by ion-exchange was studied by TPR and FTIR spectroscopy of CO adsorbed at different temperatures. Two kinds of Run+ species, differing in their reducibility, are present on the sample surface. The IR spectra of the sample and of CO adsorbed reveal the existence of a RuO2-like phase and Run+ (1 less than or equal to n less than or equal to 3) cations. The supported ruthenium is completely reduced to Ru-0 after treatment with hydrogen at 523 K. Simultaneously, a large part of the initial anatase sites are liberated for exchange. Therefore, a second ion-exchange of Ru/TiO2 is possible which allows to double ruthenium concentration. Low-temperature CO adsorption on the reduced Ru/TiO2 catalyst reveals the existence of Ru-0 sites as well as different sites typical of the bare titania. At temperatures close to the ambient one, CO interacts with the RuO clusters causing disruption of some Ru-Ru bonds and increase in dispersion of ruthenium particles. At higher temperatures, CO dissociates on particular RuO sites, thus oxidizing some Ru-0 atoms to Run+ ions. Subsequently, Run+(CO)(3) species are formed after adsorption of additional CO molecules on these latter species. Simultaneously, the titania sites that have been liberated during the reduction are re-occupied. On the Ru/TiO2 catalyst whose dispersion has been modified by CO contact, further reduction with hydrogen at 523 K leads to a new drop of the metal dispersion (similar to that of freshly reduced catalyst) showing that the metal sintering/increase in dispersion are reversible effects. When CO adsorption is performed on an oxidized catalyst, both Run+(CO)(3) and Rum+(CO)(2) species are formed on the surface. All these phenomena are also observed on the Ru/TiO2 (anatase) catalyst prepared by impregnation. The results differ only in some quantitative values.