Supported gold nanoparticles have generated an immense interest in the field of catalysis due to their extremely high reactivity and selectivity. Recently, alloy nanoparticles of gold have received a lot of attention due to their enhanced catalytic properties. Here we report the synthesis of silica supported AuCu nanoparticles through the conversion of supported Au nanoparticles in a solution of Cu(C2H3O2)2 at 300 °C. The AuCu alloy structure was confirmed through powder XRD (which indicated a weakly ordered alloy phase), XANES, and EXAFS. It was also shown that heating the AuCu/SiO2 in an O2 atmosphere segregated the catalyst into a Au–CuOx heterostructure between 150 °C to 240 °C. Heating the catalyst in H2 at 300 °C reduced the CuOx back to Cu0 to reform the AuCu alloy phase. It was found that the AuCu/SiO2 catalysts were inactive for CO oxidation. However, various pretreatment conditions were required to form a highly active and stable Au–CuOx/SiO2 catalyst to achieve 100% CO conversion below room-temperature. This is explained by the in situFTIR result, which shows that CO molecules can be chemisorbed and activated only on the Au–CuOx/SiO2 catalyst but not on the AuCu/SiO2 catalyst.