It is demonstrated that gas-phase CO pressure higher than similar to 0.1 Torr is required to segregate a sufficient amount of Pd to the surface of a well-annealed AuPd(100) sample to form contiguous Pd sites. These contiguous sites are critical in dissociating O-2 for low-temperature CO oxidation, where CO chemisorbed on Au sites clearly participates in the reaction at temperatures below similar to 400 K. Measured reaction kinetics demonstrates that the higher reaction rate is achieved on a surface with higher coverages of contiguous Pd sites.