Au catalysts have drawn broad attention for catalytic CO oxidation. However, a molecular-level understanding of the reaction mechanism on a fast time-resolved scale is still lacking. Herein, we apply in situ DRIFTS and UV-Vis spectroscopy to monitor the rapid dynamic changes during CO oxidation over Au/TiO ₂ . A pronounced transient inactivation effect likely due to a structural change of Au/TiO ₂ induced by the reactants (CO and O ₂ ) is observed at the beginning of the reaction. The transient inactivation effect is affected by the ratio of CO and O ₂ concentrations. More importantly, during the unstable state, the electronic properties of the Au particles change, as indicated by the shift of the CO stretching vibration. UV-Vis spectroscopy corroborates the structure change of Au/TiO ₂ surface induced by the reactants, which leads to a weakening of the Au catalyst’s ability to be oxidized (less O ₂ adsorption), resulting in the transient inactivation effect.