Preoxidized Au/TiO2 showed no initial activity during a first heating stage up to 70 degrees C, while prereduction yielded a high initial CO conversion at room temperature. With FTIRS, two different CO absorption bands were detected. One band is usually attributed to CO on an oxidic gold species (2151 cm(-1)), the other one is characteristic of CO on metallic gold (2112 cm(-1)). The presence of the first species appears to have a detrimental effect on the CO oxidation by O-2. The present results do not support a model in which the activity of supported gold catalysts in CO oxidation is ascribed to ionic Au particles.