It is Suggested that there may be several effects contributing to the special catalytic properties of supported nanosized gold particles, and that it is useful to order them in a hierarchy. The most important effect is related to the availability of many low-coordinated gold atoms on the small particles. Effects related to the interaction with the Support may also contribute, but to a considerably smaller extent. We base the analysis on a new set of experimental results comparing the CO oxidation rates over gold supported on different reducible and nonreducible oxides, on an analysis of a large number of published activity data, and on an analysis of density-functional calculations of the effect of metal coordination numbers in comparison to the role of charge transfer, layer thickness, and interactions with the support. (C) 2004 Elsevier Inc. All rights reserved.