The effect of halide poisoning of Au/TiO2 catalysts in low temperature CO oxidation was investigated using bromide as the poison and a combination of X-ray absorption spectroscopy (XANES and EXAFS), quantitative CO adsorption, and catalytic measurements. It was found that halide prevented full reduction of cationic Au by displacing oxyhydroxy ligands and remaining bound to An during low temperature reduction, causing a reduction in catalytic activity. On reduced Au samples, bromide (likely as NaBr molecule) was preferentially adsorbed on An and not on TiO2 and suppressed both the adsorption of CO and the catalytic activity. At low Br contents, each adsorbed Br suppressed adsorption of three CO, suggesting that Br was adsorbed on three-fold sites but the effect decreased with increasing Br content possibly due to crowding of adsorbed Br, When 5-10% of the Au was bound to Br, the catalytic activity was completely blocked, although similar to 35% of the original CO adsorption capacity remained. The data suggest that not all CO adsorption sites are catalytic active sites, and are consistent with the perimeter An atoms at/near the particle-support interface (perimeter) being active sites. (c) 2008 Elsevier B.V. All rights reserved.