Au-Cu bimetallic nanoparticles supported on SBA-15 have been reported to be much more active in CO oxidation than the corresponding monometallic counterparts in our previous work. In this work, in situ techniques including XRD, EPR, XANES, and FT-IR were employed to reveal the structural changes of the Au-Cu bimetallic particles during different pretreatment and reaction conditions, and to clarify the essence of the synergistic effect between gold and copper. The results showed that gold remained as Au(0) in any treatment conditions, while copper was very sensitive to the treatment temperature and atmosphere. The freshly reduced catalyst was a mixture of Au(3)Cu(1) intermetallic phase and Cu(2)O amorphous phase. In CO oxidation, the Au(3)Cu(1) intermetallic phase was segregated into a gold core decorated with tiny CuO(x) patches, and CO adsorbed on Au(0) reacted with active oxygen provided by the neighboring CuO(x) thus enhancing greatly the activity for CO oxidation. (C) 2010 Elsevier Inc. All rights reserved.