Highly active and stable nano-Au catalysts have been synthesized by the interaction of an Au cluster-phosphine complex with a TiO2 support. These catalysts have been characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and tested for CO oxidation. The pretreatment conditions had a profound influence on the CO oxidation activity of the Au/TiO2 catalysts. Under optimum pretreatment conditions these catalysts showed high catalytic activity. This is noteworthy, considering that previous studies (Y. Yuan, A. P. Kozlova, K. Akasura, H. Wan, K. Tsai, and Y. Iwasawa, J. Catal. (1997) 170, 19 1) on similar systems (different pretreatment) showed that CO oxidation activity was one order of magnitude lower for the Au/TiO2 catalysts. The catalyst deactivation was very slow, and studies showed that the catalyst could be successfully regenerated to regain initial activity. Diffuse reflectance infrared spectroscopy studies of CO on these supported catalysts are in excellent agreement with the infrared reflection absorption spectroscopy data obtained on model Au catalysts in our laboratory. The Au/TiO2 catalyst prepared by a temperature-programmed reduction-oxidation treatment showed high CO2 yield for the CO oxidation reaction in the presence of excess hydrogen.