The CO oxidation activity of Au/TiO2, Au/MgAl2O4, and Au/Al2O3 catalysts, all containing approximately 4 wt% Au, was compared with the detailed size and shape of the gold particles in these supported catalysts. Based on the indications that CO oxidation on Au catalysts requires low-coordinated Au atoms, the turnover frequency per An atom located at the corners of the An particles was calculated. A value of approximately 0.8 s(-1) for both the Au/TiO2 and the Au/MgAl2O4 catalyst was found. This represents an example in which the difference in catalytic activity of a Au catalyst with a reducible and irreducible support can be explained by a difference in An particle geometry only, without the need to invoke other support-induced effects. The turnover frequency per corner Au atom in the Au/Al2O3 catalyst is approximately 4 times lower, indicating that other support-induced effects not related to the reducibility may play a role in this case. (c) 2006 Elsevier Inc. All rights reserved.