Au/TiO2 is a highly active catalyst for CO oxidation when Au is dispersed as nanoparticles. However, its catalytic activity is suppressed by residual halide. Using a combination of in-situ FTIR spectroscopy, X-ray absorption spectroscopy, and microreactor studies, changes occurring at the active site upon addition of NaBr to reduced and unreduced catalyst were monitored. The active site is Au0, therefore the unreduced catalyst is inactive. Binding of bromide to Au3+ in the unreduced catalyst inhibited reduction to Au0, thereby preventing activation of the catalyst. Bromide also bound to Au0 sites on the reduced catalyst, inhibiting CO adsorption and activation, resulting in lost catalytic activity. In addition to binding to the Au particles, the bromide also mobilized them, resulting in significantly accelerated agglomeration.