Au@FeOx dumbbell nanocrystals (NCs) with different sizes of the Au domain (i.e. 4 nm, 6 nm and 10 nm) were successfully synthesized by means of a colloidal synthesis procedure. They were then deposited on gamma-alumina and tested in CO oxidation. The use of alumina minimized the contribution of the Au-support interaction in the CO oxidation, thus enabling us to clearly identify a synergy between the Au and the iron oxide that eventually resulted in an enhanced activity in CO oxidation, good sintering resistance and good stability of the catalytic performances. Among the three gold sizes tested, the dumbbells with 4 nm Au domains were found to be the most active ones. Nevertheless, we found that the activity of the 4 nm dumbbells was significantly hampered by the limited exposure of the Au or of the Au/iron oxide interface to the reacting environment. (C) 2016 Elsevier Inc. All rights reserved.