Changes in a Au/TiO2 catalyst during the activation process from an as-prepared state, consisting of supported AuOx(OH)(4-2x)(-) species, were monitored with X-ray absorption spectroscopy and FTIR spectroscopy, complemented with XPS, microcalorimetry, and TEM characterization. When the catalyst was activated with H-2 pulses at 298 K, there was an induction period when little changes were detected. This was followed by a period of increasing rate of reduction of Au3+ to Au-0, before the reduction rate decreased until the sample was fully reduced. A similar trend in the activation process was observed if CO pulses at 273 K or a steady flow of CO at about 240 K was used to activate the sample. With both activation procedures, the CO oxidation activity of the catalyst at 195 K increased with the degree of reduction up to 70% reduction, and decreased slightly beyond 80% reduction. The results were consistent with metallic An being necessary for catalytic activity.