CO oxidation at room temperature over the Au/CeO 2 catalyst was investigated, of which the induction period arising from unreduced cationic Au was disclosed. The effect of humidity in reactant stream on catalyst activity was investigated during induction and deactivation periods. The induction period was significantly reduced from 44.8 min in dry stream to 3.3 min in wet stream containing 0.58 vol % of H 2 O. The induction period was slightly shortened by further increasing water concentration in the reactant stream. The maximum initial activity was achieved with a wet stream containing 0.74 vol % of H 2 O vapor. In situ DRIFTS measurements suggested that humidity contributed to the formation and consumption of reaction intermediates [COOH] s and hence enhanced the initial activity. After the induction period, the deactivation phenomena of Au/ CeO 2 catalyst at varied rates under both wet and dry streams were observed. In contrast to a slow deactivation under dry stream, a fast deactivation in wet stream was first observed due to rapid blockage of active sites by water adsorption, followed by a slow deactivation rate due to gradual formation of surface carbonate species over catalyst with time on stream. A mechanism was proposed to illustrate the existence of induction period under both wet and dry streams.