Supported Au catalysts are generally prepared from chloride-containing An precursors and their properties are highly sensitive to preparation procedures. To better understand and control the variables important in the preparation of these catalysts, the effect of chloride on the catalytic performances of highly active Au/Al2O3 catalysts in the selective catalytic oxidation of CO (SCO) was probed. The complex manner in which Cl- affected An catalysis was deconvoluted in a series of complementary experiments where chloride was quantitatively removed and added. The residual chloride was found to affect the activities in two different ways. It facilitates the agglomeration of An particles during heat treatment, and it inhibits the catalytic activity by poisoning the active site. Interestingly, chloride poisoning is not only observed at the steady state, it is in fact more pronounced in the transient toward steady state. In order to better assess the poisoning effect of Cl-, phosphate was introduced to the support surface as a postsynthesis treatment before the addition of Cl- in some experiments. The results showed that activity suppression was observable at a Cl-/Au molar ratio as low as 0.0006. Thus a very small fraction of Au is associated with the active site. This implies that the activity per active site is extremely high.